At the beginning of the play (Act 1, Scene 2) we learn that Macbeth is fiercely loyal and a brave, dauntless, fighter. He will give everything to save his beloved Scotland from the insurrection headed by the Norwegian king, the traitorous Macdonwald, and the thane of Cawdor. An injured soldier gives King Duncan the following report:
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
This fearless, loyal image is, however, soon replaced by the image of an overly ambitious, gullible character who is more than readily impressed by the forces of darkness. When he and Banquo meet the witches, the witches greet Macbeth with the title "thane of Cawdor" and tell him that he will be "king hereafter." Macbeth is clearly overwhelmed by what they say and is later enraptured when their first prediction is confirmed as true. Macbeth states in an aside:
Glamis, and thane of Cawdor!
The greatest is behind.
This quote indicates Macbeth's ambition. He states that gaining the title of Cawdor means that he has overcome the greatest hurdle in his desire to be king. More titles mean greater power, and it will be easier for him to lay claim to the throne should the occasion arise. His desire is further confirmed when he later says, again in an aside:
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme.
Macbeth is in awe about the witches' predictions and believes that the confirmation of their first prediction must mean that the others will naturally follow. This shows us how gullible he is. His ambition controls his ability to reason. Even Banquo notices that he is enraptured and comments "Look, how our partner's rapt."
Lady Macbeth later makes it clear that Macbeth is not a ruthless man. She states, in Act 1, Scene 5, that he is:
too full o' the milk of human kindness
To catch the nearest way
This means that Macbeth is kindhearted and gentle and does not have the inner malice, in spite of his ambition, to perform regicide. He is an honorable man and will not deliberately mislead others, as Lady Macbeth says:
what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
Macbeth's lack of malice is informed by his desire to withdraw from committing a foul act such as killing a king. He finds reasons not to proceed. This further proves his loyalty, respect for authority, and values, as expressed in the following soliloquy from Act 1, scene 7, shows:
He's here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself.
Lady Macbeth's sentiments are confirmed later when we learn how overwhelmed Macbeth is by the enormity of the deed he is about to commit. He lacks the callousness to commit murder and is so stressed out and pressured that he starts hallucinating. He is definitely not a coldhearted assassin and is surely not cool, calm, and collected, as the following extract proves:
...art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
He imagines seeing a bloody dagger float in front of him. We also discover that he is deeply superstitious, for he sees signs and portents in everything. For example, he believes that his inability to say "amen" after he killed Duncan is significant. He believes that he has heard voices telling him that he has "murdered sleep" and that he "shall sleep no more."
Once the deed is committed we learn of Macbeth's deep remorse. This ties in with his nature as a gentle person. Macbeth is totally overwhelmed by what he has done and believes that his guilt will never fade.
Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas in incarnadine,
He is assured by his wife, though, that it is not an issue he should be too concerned about.
As the play progresses, Macbeth loses all of his goodness and integrity. He quickly devolves into a ruthless, murderous tyrant. His further degradation becomes apparent soon after Duncan's murder. First, he is deceitful when he lies about why he killed Duncan's guards and claims that his vengeance was out of love for Duncan:
...who could refrain,
That had a heart to love, and in that heart
Courage to make 's love known?
Once he is ensconced on the throne he becomes corrupted by the power his position yields. He becomes paranoid and feels insecure. As a result, he plots Banquo's assassination and states, in Act 3, Scene 1:
To be thus is nothing;
But to be safely thus.—Our fears in Banquo
His guilt at killing his friend overwhelms him, though, and he imagines seeing Banquo's ghost. This, however, does not stop him from committing further bloodshed, and he sends out assassins to kill whomever is against him. He becomes more bloodthirsty and has Macduff's entire family and his servants assassinated.
By Act 4, we know that Macbeth has become so malevolent that even the witches recognize his malice. When they hear him arrive they comment, "Something wicked this way comes." At this point, they also make him believe that he is invincible. The witches predict, firstly, that "none of woman born" will be able to harm him and then that only when Birnam wood comes to Dunsinane shall he be vanquished. He cries out:
Then live, Macduff: what need I fear of thee?
Who can impress the forest, bid the tree
Unfix his earth-bound root? Sweet bodements! good!
In the end, though, Macbeth is honest enough to admit his foolishness when he realizes how the witches misled him through paradox and equivocation. He states, in Act 5, Scene 5:
I pull in resolution, and begin
To doubt the equivocation of the fiend
That lies like truth:
When he confronts Macduff in a fight to the death, he finally accepts just how much he has been deceived:
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
And be these juggling fiends no more believed,
That palter with us in a double sense;
He curses the witches for their lies, for he has just heard that Macduff's birth was not natural but that he was removed from his mother's womb by Caesarean section. He refuses to fight anymore but also refuses to surrender. In a final act of courage and honor, he challenges Macduff to a battle:
Yet I will try the last. Before my body
I throw my warlike shield. Lay on, Macduff . . .
Macduff kills him and then presents his head to all and sundry.
Most of the information a reader learns about Macbeth during the course of Shakespeare's Macbeth is revealed, rather than directly spoken. So when you ask for quotes that actually describe Macbeth's character, rather than reveal it, you're eliminating most of the play. In other words, most of the characterization in the play is shown, rather than told. That said, I'll mention one line for you.
In Act 4.1, after the witches have prepared a nasty brew, presumably for Macbeth to drink so he will see visions, just before Macbeth enters the scene, the Second Witch says:
Something wicked this way comes. (Act 4.1.45)
That accurately states Macbeth's character. He is wicked.
I'll mention one other line briefly. Referring to Macduff failing to attend Macbeth's feast, Lennox says:
His presence at the tyrant's feast,... (Act 3.6.21-22)
He refers to Macbeth as a tyrant. That is also direct characterization.