Brutus is a man spoken of by others in the play as honorable. This term is one you could measure his actions in each Act against (as many critics have done) to see if you find him to be consistently an "honorable" man. That term would be one to consider including to describe his behaviour in all five Acts.
In Act One, I would describe Brutus as both cautious and mysterious. He appears cautious because he listens to Cassius and all his arguments, but still tells him that he will consider his words, but will do so at his own leisure (I, ii, 167-169):
. . .What you have said
I will consider. What you have to say
I will with patience hear., and find a time
Both meet to hear and answer such high things.
And yet, he is also mysterious since he spends this entire scene listening to Cassius and Casca's very bald opinions of Caesar and the crown that has been offered him, but does not commit his own voice to the dissent. However, he does agree to meet with Cassius later, so he is also interested.
In Act II, he is decisive. He has a soliloquy that begins "It must be by his death," and, once the Conspirators arrive at his house, he wastes no time in making all of the rules about how they will proceed. This shows he is also respected by all the other Conspirators, even Cassius. He should be noted as just and realistic too, since he adamantly opposes killing Antony, reasoning that they will seem "too bloody," if they do.
In Act III, he is gullible, falling for Antony's ruse and allowing him (over Cassius' protest) to speak over Caesar's body at the funeral. He is also calm and reasonable in his funeral oration, claiming that he acted out of patriotism.
In Act IV, he is dismissive of Cassius and his complaints. He is also accusatory, telling Cassius that he is "condemn'd to have an itching palm." These two qualities together might add up to considering Brutus in this Act as being stubborn and obstinate.
And in Act V, he is resigned to following through on what appears to be a losing battle. He is grief-stricken over Cassius body:
. . .Friends, I owe more tears
To this dead man than you shall see me pay.
And finally he is resolved to end his life and do so with dignity. He says (V, v, 33)
. . .Countrymen,
My heart doth joy that yet in all my life
I found no man but he was true to me.
I shall have glory by this losing day
More than Octavius and Mark Antony
By this vile conquest shall attain unto.
And so he dies hopeful that his legacy will leave behind him a better name for posterity than either Antony or the man who would become Emperor of Rome, Octavius.
For more opinons on Brutus, please follow the links below.