a lack of scruples. In act IV, scene iii, we learn that Cassius is taking bribes and is asking Brutus to allow his friend to take to do the same. Brutus is upset that Cassius honors corruption and begins to question whether or not Cassius had good intentions in organizing Caesar's assassination:

What, shall one of us,That struck the foremost man of all this worldBut for supporting robbers, shall we nowContaminate our fingers with base bribes...? (22-25)

Finally, in act V, scene iii, Cassius shows the reader that he is ultimately a coward. Instead of being captured and facing his punishment for killing Caesar, he chooses to take his own life. Even in suicide he is a coward because he orders his slave Pindarus to commit the act and asks him to wait until his master's face is covered and to use the sword "that ran through Caesar's bowels" (44). He could not face his punishment, could not face death with open eyes, and could not stab his own chest, demonstrating his excessive cowardice.

Julius Caesar, as Shakespeare presents him, is a powerful, charismatic leader, and brilliant military tactician, whose victories have inspired "the masses" with pride in Roman rule. He is able to compel loyalty, to a fanatic extent, in those under his command.

But, although Shakespeare is at pains to suggest ambiguity in this situation, we are led to believe that he may, in fact, aspire to become a dictator, a supreme ruler whose populist support does not reflect the ideals of the Roman republic. This is why Brutus and the other conspirators fear him and eventually assassinate him. Caesar's success in his military campaigns, which rely on his "top–down," absolute rule, leads him to believe that this system is also the best way to govern a country, or, in this case, an empire. He has not grasped the idea that "absolute power corrupts absolutely."

Caesar's greatest strength, as Shakespeare emphasizes in the play, is his willpower. According to Plutarch in his "Life of Julius Caesar," Caesar had truly remarkable determination and willpower. When Caesar decided to do something, he did it, as illustrated by his conquest of Gaul, which took eight years. Caesar also had the ability to command the loyalty of his soldiers. They would do anything for him. They were the source of his strength and support as a Roman politician as well. Plutarch writes that Caesar was responsible for the deaths of two million people and sold many others into slavery. In one incident recorded in his own book The Gallic Wars, Caesar sold a whole city of 50,000 men, women, and children as slaves because they initially refused to surrender to him. 

Caesar's weaknesses included physical ailments. As Shakespeare shows in his play, Caesar suffered from epileptic seizures and had one of them while attending the Lupercal games. Caesar's ambition, or hubris, was probably his main weakness. Shakespeare portrays Caesar as a supreme egotist, and it was this ambition and egotism that led to his assassination. 

Mark Antony's strengths include being a strong orator.  He is able to convince the crowd to turn against the conspirators and create a civil war with his "Friends, Romans, Countrymen" speech.  He is a loyal friend to Julius Caesar and avenges Caesar's death.  After telling Cassius and Brutus that he would gladly join them if they could explain why Caesar had to be killed, once Antony is alone with Caesar's body, he asks for forgiveness with, "Oh pardon me, thou bleeding piece of earth/That I am meek and gentle with these butchers!"  He does, however, prove to be more ambitious than Cassius believed Caesar to be.  Once the civil unrest began, Antony got together with Octavius and Lepidus to create a triumverate.  Once things got going his way, though, he planned on getting rid of Lepidus.  Antony was even ready to sacrifice "his sister's son"  for his political gains.

Brutus' strength is that he is a loyal Roman above all else: before friendship, before family, before self.  He joins the conspiracy because he is convinced by Cassius that Caesar is overly ambitious.  In his oration after the assassination, he shows that he did care for Caesar with his words: "Because Caesar loved me, I weep for him....but as he was ambitious, I slew him".  His major fault is that he is a bad judge of character. He believes that Antony is harmless, but soon learns differently.

Shakespeare seems to have held the opinion that all men are mixtures of good and bad. He shows this clearly in Julius Caesar. Caesar is a great man, but he is an egomaniac. Antony is courageous and resourceful but two-faced. Cassius is brave, intelligent, and an inspiring talker, but he is greedy and stingy. Brutus, according to Mark Antony, had an exceptionally fine character. At the end of Act V, Scene 5, Antony says of his defeated enemy:

This was the noblest Roman of them all.All the conspirators save only heDid that they did in envy of great Caesar.He only in a general honest thoughtAnd common good to all made one of them.His life was gentle, and the elementsSo mixed in him that nature might stand upAnd say to all the world "This was a man."

Yet Brutus had his weaknesses. He seems to be the kind of introverted, solitary man who is highly intelligent and learned but lacks common sense. He judges others by himself. Once he takes the leadership of the conspiracy, he makes terrible blunders. Mark Antony practically makes a fool of him in getting permission, against Cassius' advice, to speak in Caesar's funeral and then delivering a speech which is far more effective than Brutus' formal speech of self-justification. Cassius even wanted to assassinate Antony along with his great friend Caesar, but Brutus overruled him in Act II, Scene 1, saying:

Our course will seem too bloody, Caius Cassius,To cut the head off and then hack the limbs,Like wrath in death and envy afterwards--For Antony is but a limb of Caesar.Let's be sacrificers, but not butchers, Caius.

It is ironic that when Brutus and Cassius are having their acrimonious argument in the tent in Act IV, Scene 2, Brutus says:

I did sendTo you for gold to pay my legions,Which you denied me. Was that done like Cassius?Should I have answered Caius Cassius so?

That was done exactly like Cassius. Cassius loves gold, and he has the worldly wisdom and aggressiveness necessary to acquire it, even though their armies are forced to withdraw to the countryside while Antony and Octavius have free access to all the resources of Rome. Brutus judges others by himself. He is noble, generous, and unselfish, and he expects others to be like himself. He was mistaken about Antony, and he is mistaken about Cassius, as he discovers when they have their falling out in his tent. Brutus has to ask Cassius for gold because he is too noble, too aristocratic, too genteel to get it the hard way (which is just about the only way to get it). As he tells Cassius:

For I can raise no money by vile means.By heaven, I had rather coin my heartAnd drop my blood for drachmas than to wringFrom the hard hands of peasants their vile trashBy any indirection.

Brutus shows a further weakness in judgment when he decides to fight Antony and Octavius at Philippi against the strong advice of Cassius. Brutus is not cut out to be an assassin and a revolutionary. He is a meditative, scholarly, impractical type of man who is misled by the crafty, ambitious, and unscrupulous Cassius into becoming the leader of the bloody coup. Cassius might have been a better leader, but Brutus was loved and respected by the Roman people, whereas Cassius was not liked because he was not a likable person--and apparently he knew it.

Brutus is a great character.  He is a loyal friend, a trusted confidant, and a noble character whom all respect.  Otherwise, the conspirators would never have bothered with recruiting him and manipulating him with falsified letters to make him feel the need of his leadership.  If Brutus has a fault, it is his sincere love of country or his gullible nature.  He does not immediately take up the conspirators' cause, but he does, after much agonizing, agree that Caesar must die if he is so ambitious as to accept the crown and become emperor for an indefinite period of time.  Before this time, Roman law stated that one man may take the role of emperor in times of great strife, but only for the period of six months at which time he would be expected to step down.  Brutus was led to believe that Julius Caesar would not agree to step down, and all that is Rome would be lost. 

Brutus is a strong, noble, and honorable character.  He is not the best example of friendship, but he is the best example of loyalty to one's country in the play. 

In Shakespeare's Julius Caesar, it is ironic that in Act I Cassius tells Brutus

Men at some time are masters of their fates:

The fault, dear Brutus, is not in our stars,

But in ourselves, that we are underlings. (1.2.145-147)

For, Cassius is at times master of his fate, and at others is himself superstitious and weak. 

In Act I, Scene 2, from which these lines come Cassius persuasively convinces Brutus to join the conspiracy to assassinate Caesar.  In Act III after the men have slain Caesar, it is Cassius who recognizes the threat that Marc Antony poses; he advises Brutus to have Antony killed.  But, Brutus tells him that they will make a friend of Antony.  Wisely, Cassius replies that his doubts always turn out to be justified,

I wish we may.  But yet have I a mind

That fears him much; and my misgiving still

Falls shrewdly to the purpose. (3.1.158-160)

Then, when Brutus gives Antony permission to address the Romans after he does, Cassius warns him against doing so:  "You know not what you do" (3.1.250),  His suspicions of Antony are correct, but he defers to Brutus.  Still, he expresses his anxiety, 

I know not what may fall; I like it not he defers to Brutus. (3.1.262)

Of course, Marc Antony turns against them and becomes their mortal enemy, fomenting a civil war, and defeating them at Philippi.  Even there, Cassius's assessments are correct, for he suggests that his and Brutus's troops to remain at Sardis and force the others to advance so that they will be fatugued and use valuable resources, but Brutus disagrees.  As it turns out, Cassius again is correct, although he has acquiesced to Brutus.

But, just before this final battle, Brutus and Cassius quarrel bitterly.  Cassius accuses Brutus of wronging him repeatedly, and complains weakly that Brutus no longer loves him:

Hath Cassius lived

To be but mirth and laghter to his Brutus

When grief and blood ill-tempered vexeth him? (4.3.124-126)

just as he has been worried in the first act: 

Brutus, I do observe you now of late;

I have not from your eyes that gentleness

And show of love as I was wont to have (1.2.36-38)

Finally, Cassius becomes weakly superstitious.  In Act 5 he talks to Messala, telling him that even though he "held Epicurus strong" and has not believed in omens, now he has seen

Two mighty eagles fell, and there they perched,

Gorging and feeding from our soldiers' hands,

Who to Philippi here consorted us.

This morning are they fled away and gone,

And in their steads do ravens, crows, and kites

Fly o'er our heads and downward....

Our army lies, ready to give up the ghost. (5.1.87-93)

In the end, Cassius still has wisdom, but he is fearful and superstitious, and defers to Brutus. 

________________

One of the triumvirs after the death of Caesar, Lepidus is considered unworthy to be one of the three rulers of the Roman empire by Marc Antony, while, on the other hand, Octavius is willing to honor him as "a tried and valiant soldier" (4.1.32).   At the beginning of Act IV, the triumvirate compile a death list of their political enemies.   Lepidus is sent to get Caesar's will so they can reduce some of the legacies in it; when he returns, with a lack of honor, Lepidus  consents to allow his brother to die provided Antony will sacrifice his nephew.  So, while he may be an excellent soldier, Lepidus lacks loyalty to his family.

In Shakespeare's play, Julius Caesar, Cassius is able to scheme and manipulate, and is a good judge of character. This might be considered a strength as he is able to convince people to do what he wants. However, he is weak in that his motivation is based mostly on self-interest and jealousy.

In Act I, scene ii, Cassius tries to convince Brutus to turn against Caesar and become a part of the plot against him, as seen in this, the "seduction scene:"

CASSIUS:

Brutus, and Caesar: what should be in that Caesar? Why should that name be sounded more than yours? Write them together, yours is as fair a name; Sound them, it doth become the mouth as well... (I, ii, 148-151)

Here Cassius is telling Brutus that he is just as good as Caesar; why should people call out Caesar's name more than Brutus' as they are both equally as good. In this quote we can see that Cassius is a good judge of character, as he would never have approached Brutus if he thought the plot would be betrayed by the man.

The following quote exemplifies Cassius' jealousy...

CASSIUS

Caesar cried, “Help me, Cassius, or I sink!” I, as Aeneas our great ancestor Did from the flames of Troy upon his shoulder The old Anchises bear, so from the waves of Tiber(120) Did I the tired Caesar. And this man Is now become a god, and Cassius is A wretched creature, and must bend his body If Caesar carelessly but nod on him. (I, ii, 117-124)

In this quote Cassius recalls Caesar calling for him when he was drowning in the Tiber River, and Cassius carried him out and saved him. Now, Cassius laments, Caesar has become like a god, and Cassius is nothing but a "wretched creature," who has to bow if Caesar (who owes his life to Cassius) casually nods at him.

After Caesar's death, Lepidus is a triumvir (one of three rulers). Whereas Octavius wants to honor his service as a soldier, Antony does not believe Lepidus is good enough to be one of three rulers of Rome.

In terms of his weaknesses, Lepidus is an old man; he has very little power even though he is one of the leaders of the Roman Empire. He "lacks worldly wisdom," but, as Octavious points out, he is loyal and was a brave soldier.

Antony shares his feelings regarding Lepidus in the following passage:

ANTONY:

This is a slight unmeritable man, Meet to be sent on errands. Is it fit, The three-fold world divided, he should stand One of the three to share it?  (IV, i, 13-16)

Antony questions the legitimacy of Lepidus' important role in the government when he is really only good for running errands.

Octavius defends the old man, Lepidus:

OCTAVIUS:

You may do your will, But he's a tried and valiant soldier. (IV, i, 30-31)

Having known Lepidus for a long time, Octavius notes that he is a brave soldier with a great deal of experience on the battlefield. He points out that Antony thought better of Lepidus when it suited him.

As with all people, each has strengths and weaknesses. It would seem that Lepidus is the more honorable man, and Cassius less honorable and more self-interested.

Additional source:

http://www.enotes.com/julius-caesar-text/act-iv-scene-i