What evidence from Julius Caesar illustrates the themes of power and friendship?

The theme of power is dealt with early in the play, as Caesar and Brutus discuss the danger of Caesar’s ambition in Act I. In Act II Brutus has a soliloquy that addresses his inner conflict over how to deal with Caesar:

The abuse of greatness is when it disjoins
Remorse from power, and, to speak truth of Caesar,
I have not known when his affections sway'd
More than his reason. But 'tis a common proof
That lowliness is young ambition's ladder,
Whereto the climber-upward turns his face;
But when he once attains the upmost round,
He then unto the ladder turns his back,(25)
Looks in the clouds, scorning the base degrees
By which he did ascend.

Here, Brutus presents a philosophy of power. It becomes dangerous when it is separated from “remorse,” or the inclination to use power conscientiously. He is also saying that it’s well known that someone who seems to spurn power, once attaining it, “turns his back” on those who helped him rise to power.

Brutus and Caesar establish a solid, if imperfect, friendship in the play. Early in the play, Cassius notes that Brutus does not seem to see him as a friend anymore. Brutus responds, saying it’s not true, it’s just that he’s had a lot on his mind lately. Later, after Caesar’s death, Brutus and Cassius are preparing to assault the armies of Antony and Octavius. They have a sharp disagreement, following by this scene of reconciliation:

BRUTUS
When I spoke that, I was ill-temper'd too.

CASSIUS
Do you confess so much? Give me your hand.

BRUTUS
And my heart too.

CASSIUS
O Brutus!

BRUTUS
What's the matter?

CASSIUS
Have not you love enough to bear with me,
When that rash humour which my mother gave me
Makes me forgetful?

BRUTUS
Yes, Cassius; and, from henceforth,
When you are over-earnest with your Brutus,
He'll think your mother chides, and leave you so.

Students are often somewhat uncomfortable with this scene, as it seems that the characters express their emotions in a very straightforward manner—a manner in which men do not speak to each other anymore.

How is the theme of friendship portrayed in Julius Caesar?

Both Brutus and Mark Antony are close friends of Julius Caesar, but they have completely different ways of showing their friendship. Despite his professed love for Caesar, Brutus willingly joins the assassination plot against the Roman dictator. Not only that, but he plunges a knife right into his old friend, believing that by doing so he will save the Republic from Caesar’s tyrannical ambitions. In other words, Brutus, in killing Caesar, has put his city before his friendship. When push comes to shove, the Roman Republic is more important to Brutus than Caesar.

Mark Antony also loves Caesar, but he shows his love in a much healthier, more conventional way. Though initially appearing to go along with the assassins, he quickly turns the table on them, mounting the rostrum in the Roman Forum to give an unforgettable speech that will turn the masses against Brutus and his band of assassins. In his own speech, Brutus had convinced the mob that, though he loved Caesar, he loved Rome more.

But when Mark Antony steps up to the rostrum he shows what real love and real friendship are all about. He shows his love for his dear departed friend not just by heaping praise upon him, but by reminding the Roman plebs how much Caesar loved and cared for them. Once he’s done that, once he has the audience firmly in the play of his hand, he’s in a position to incite the mob against the assassins, thus providing the catalyst for the terrible civil war that will soon follow. In doing so, Mark Antony has proved his love for Caesar beyond any doubt.

What is the role of friendship in Julius Caesar?

Friendship is hugely important in "Julius Caesar", not least as an oil which can be used to slick along political manipulation. Yet I'm not too sure how "real" the friendships in the play turn out to be.

Right at the start, to persuade Brutus on board, Cassius appeals to their friendship, as well as making clear how great he thinks Brutus. So, Brutus has to respond personally rather than just professionally: which gives Cassius the cue he needs:

BRUTUS
But let not therefore my good friends be grieved—
Among which number, Cassius, be you one—
Nor construe any further my neglect
Than that poor Brutus with himself at war
Forgets the shows of love to other men.

What does Cassius say when he turns up at Brutus' orchard with some of the conspirators? He makes it very clear that the men are known to Brutus, admirers of Brutus', and Brutus' friends:

BRUTUS:
I have been up this hour, awake all night.
Know I these men that come along with you?

CASSIUS:
Yes, every man of them, and no man here
But honors you, and every one doth wish
You had but that opinion of yourself
Which every noble Roman bears of you.
This is Trebonius. 

Moreover, in the famous quarrel scene in Act 4, Cassius and Brutus really argue with each other in the way that only two people who know each other can. Yet, tellingly, at the very end of the play, when Cassius names his best "friend", it isn't Brutus, but Titinius:

CASSIUS
O, coward that I am, to live so long,
To see my best friend ta'en before my face!

There are lots of other friendship to look at: Antony and Octavius, Antony and Caesar, Cassius and Casca, Brutus and Metellus Cimber. Yet I think you'll see that most of them, like most else in this play, have a political underbelly.

Discuss the theme of friendship in Julius Caesar.

One way to discuss the theme of friendship in Julius Caesar is to look at statements of friendship versus actions of friendship.

Cassius makes many statements of friendship throughout the play, statements which contrast his ambitions. Antony, contrastingly, does not speak of friendship but instead speaks of honor. Though his own motives are debatable, Antony's actions suggest a real bond of loyalty to Caesar where Cassius' proclamations of affection ring false in light of his actions.

This is a big topic, and you might like to narrow it down to make your analysis more pointed and focussed. However, I will give you a few remarks to get you started as you gather your thoughts.

This play presents us with a world that is above all about the cut and thrust of politics, in every sense of these words. In this sense, friendship is a luxury that is often sacrificed in the face of political advantage and the desire for power. One of the most chilling examples of this is the way in which, in Act IV scene 1, Antony and Lepidus eerily mimic the conspirators earlier on in the play as they talk over who will be killed and who will survive in their own movement to gain power and control over Rome and the empire, before Antony turns very quickly on one of the Romans with whom he shares power:

This is a slight unmeritable man,
Meet to be sent on errands; is it fit,
The threefold world divided, he should stand
One of the three to share it?

Even though Antony has made a political alliance with Lepidus and Octavius, this "friendship" is shown to be based on nothing more than temporary convenience, and he is quick to challenge this alliance and suggest to Octavius that they should split the power between them alone rather than divide it three ways. Through such methods, Shakespeare presents us with a rather chilling impression of personal relations in this cloak-and-dagger world.

Explain the theme of friendship in The Tragedy of Julius Caesar.

It's interesting that you'd see a theme of friendship in the story.  I would say most of the so-called friendships are more of loosely-knit alliances of convenience.  Brutus is not truly a friend of Cassius, even though Cassius professes his friendship to him.  Likewise, Cassius is simply using Brutus to add pull to his growing conspiracy; he feels people would trust Brutus' explanation of why Caesar had to die, rather than his own or that of one of the other conspirators.

On the other side of the fight, you have Mark Antony, who is a so-called friend of Caesar's.  He comes "not to praise him, but to bury him" in his spirited requiem, yet proves he did love the man.  However, he is no true friend of Ocatavius, the heir to the throne.  Instead, Octavius, Caesar's nephew, has a power struggle with Mark Antony to determine who should truly take command out of the three men fighting the conspiracy. 

There are few friendships, and even fewer people anyone can trust in this tragedy.  However, it's an interesting theme, and can be investigated through a careful observation of the interactions of Caesar and Mark Antony.