In Julius Caesar, when Caesar espies Cassius, he becomes anxious about his power being threatened; he tells Antony,
Let me have men about me that are fat,
Sleek-headed men, and such as sleep a-nights.
Yond Cassius has a lean and hungry look;
He thinks too much: such men are dangerous....Such men as he be never at heart's ease
Whiles they behold a greater than themselves (l.2.198-215)
In Act I, also, Casca tells Brutus that Caesar refused a crown three times when Marc Antony offered it to them.
...I saw Mark Antony offer him a crown--and, as I told you, he put it by once; but for all that, to my thinking, he would fain have had it. Then he offered it to him again; then he put it by again; but to my thinking, he was very loath to lay his fingers off it. And then he offered it the third time. He put it the third time by; and still as he refused it, the rabblement hooted, and clapped their copt hands, and threw up their sweaty nightcaps....(1.2.241-250)
If Caesar were not hungry for power, he would not make such a show of refusing the crown, Casca argues. Casca states that Caesar has fallen in the market place, but
before he fell down, when he perceived the common herd was glad he refused the crown, he plucked me ope his doublet and offered them his throat to cut.....When he came to himself again, he said, if he had done or said anything amiss, he desired their worships to think it was his infirmity.(1.2.267-274)
Cassius contends that Caesar's theatrics are meant to trick the people, in his desire to win the people over in his desire for property. Cassius blames the people for allowing Caesar to become so powerful:
...if Caesar had stabbed their mothers, they would have done no less. (1.2.277)
Anxiety about anyone else who looks eager for advancement, dramatic about being the "mighty Caesar" of the people--these are the arguments that Cassius and Casca use to convince Brutus that Caesar desires to be powerful, words to convince Brutus that Caesar will become a tyrant.
In Act 2.2 of Shakespeare's Julius Caesar, Caesar at one point decides not to go to the Senate today. His wife doesn't want him to go, and a dream she has seems to be a bad omen. The conspirators are planning to assassinate him on his way to the Senate, of course, so they need Caesar to go.
Decius accomplishes this by appealing to his ambition. He tells Caesar:
...the Senate have concluded
To give this day a crown to mighty Caesar.
If you shall send them word you will not come,
Their minds may change. Besides, it were a mock
Apt to be rendered, for someone to say
"Break up the Senate till another time,
When Caesar's wife shall meet with better dreams."
If Caesar hide himself, shall they not whisper
"Lo, Caesar is afraid"?
Pardon me, Caesar, for my dear dear love
To your proceeding bids me tell you this,
And reason to my love is liable. (Act 2.2.97-109)
Caesar, convinced, answers:
How foolish do your fears seem now, Calphurnia!
I am ashamed I did yield to them.
Give me my robe, for I will go. (Act 2.2.110-112)
This demonstrates Caesar's ambition. The thought of being crowned is just too much to pass up. This also shows that when Caesar three times rejected the crown offered him by Antony, he was just posturing and appearing to reject it. He wants the crown, or he would not be so quick to change his mind and go to the Senate in the above passages.