Brutus is reluctant at first to conspire against Julius Caesar and would not have gotten involved if Cassius hadn't made such a strong effort to involve him. Cassius was well aware that he was not personally liked or respected himself. He wanted Brutus to look like the leader, although he did not really want him to lead. He thought he could be the actual leader himself and manipulate Brutus. But once Brutus had committed himself to the conspiracy, all the other conspirators looked to him as the de facto leader of their plot, and there was little that Cassius could do to assert his own authority even though he was the one who had conceived and initiated the conspiracy in the first place.
Cassius was a realist. Brutus was an idealist, a philosopher, an introvert, a deep thinker. He is often shown alone, whereas Cassius is invariably in the company of others. Cassius is bound to regard Brutus as an impractical dreamer, while Brutus is bound to regard Cassius as a schemer and a petty opportunist. The main change that takes place in Brutus's character is that he becomes more and more stubborn about having his own way. He doesn't listen to Cassius--but he probably doesn't listen to anybody else either. It begins to look as if the conspiracy would only be getting rid of one autocrat for another. Caesar might have made a better king than Brutus because Caesar was more realistic and practical. Brutus seemed to think that everybody was an idealist and patriot like himself. He didn't understand that most men were more like Cassius than they were like Brutus. Most men are greedy and selfish. Brutus was idealistic and honorable. This was probably because of all the serious literature he had been reading all his life. He was bookish and scholarly. Hamlet is a similar type of character.
Cassius ends up in a subordinate position. He cannot do without Brutus, but he knows that Brutus can get along very well without him because everyone naturally looks to him as the political and military leader. These two men were probably not that different in reality, but Shakespeare took pains to make them conspicuously different because he did not want them to seem like twins. Brutus can best be understood by contrasting him with Cassius, just as Cassius can best be understood by contrasting him with Brutus.
Brutus develops from being a cautious, over-idealistic man to one who starts taking a bit more risks as he attempts to cope with the disastrous after-effects of the killing of Caesar. Early in the play we see the philosophical, analytical side to his nature as he agonises at length with himself over whether he should join the conspiracy against Caesar or not. However, in the latter stages, when forced to join battle with Antonyand Octavius after the assassination, he starts to become a bit more reckless – certainly by his own former standards. This is seen most clearly when Cassius counsels staying where they are rather than advancing to attack the enemy at Philippi. Brutus at this point throws all caution to the winds, insisting that they be proactive rather than wait tamely for battle:
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Brutus here is saying to seize the moment, to act, to plunge into the current rather than hang back out of fear and later regret losing their chance. He therefore appears quite different from the hesitant man of the play’s opening scenes.
There is more than a hint of desperation in Brutus at this point too, however. We have seen Caesar’s ghost previously telling him that they will meet again at Philippi; it may be that really he is resigned to his fate and just wants to bring on the moment of his death as quickly as possible and get it over with.