In seeking differences and similarities in the first two acts of "Julius Caesar," the reader would do well to examine closely the main characters as they are introduced in the exposition. Also, the element of the supernatural is present in these two acts, so the reader should analyze the effects of supernatural forces and superstition upon the characters.
For instance, in Act I as Caesar triumphantly traverses the streets of Rome, a soothsayer warns him to "Beware the Ides of March." However, the vain Caesar dismisses the man as a "dreamer," and he continues his entry into Rome. In contrast to this dismissal of superstition, Caesar observes Cassius and recognizes the "lean, hungry" man as a threat; still, his conceited nature causes him to not be unconcerned, thus making him vulnerable as Cassius tries to elicit Brutus in a plan against Caesar, contending that Caesar will make himself king.
In Act II, Brutus, like Caesar, becomes vulnerable. He is "seduced" by the words of Cassiius and the forged letters sent him about Caesar. However, rather than his vanity and conceit, it is Brutus's idealism that prevents his understanding of the situation. And, again superstition and supernatural forces play an important role as Caesar, because of his superstitious nature, is initially persuaded by Calpurnia, his wife, not to go to the senate, but later persuaded by his vanity and conceit when ---comes. Likewise, Brutus is persuaded by Portia who relates her dreams from the previous night, but the interruption of others causes him to remain with the conspiracy to kill Caesar.
Certainly the intent and sophism of Cassius displayed in Act I in his speech--
Men at some time are masters of their fates:/The fault, dear Brutus, is no in our stars,/But in ourselves that we are underlings (I,ii,140-142)
--and soliloquy are in great contrast to the soliloquy in Act II of the ideal Brutus, who perceives Caesar as a threat to the state of Rome, one who should be removed for the greater good.