Julius Caesar readers are now moving on from the first three acts to Acts IV and V, which just aren't very satisfactory and have the feeling of the play falling apart. (MacBeth has the same fault). Acts I through III are neat and tightly plotted. The conspirators conspire, Brutus is brought into the plot, Caesar is killed, Brutus tries to justify the assassination to the crowd, but Mark Antony fires them up with one a the greatest pieces of oratory ever written, and the mob goes on the rampage. The characters have recognizable personalities. Caesar is the most pompous and arrogant embodiment of hubris and/or ego ever to walk the stage.* Cassius is simply Iago (we read Othello right before Julius Caesar). Casca is a sneering cynic. Antony is a rabble-rousing demagogue, who nonetheless seems deeply devoted to Caesar for some undefined reason. And Brutus -- well Brutus is the perfect demonstration of why the man of flawless character and irreproachable integrity that conspirators want as their figurehead should not be involved in the sordid, squalid business of actually conspiring. He makes consistently bad decisions while Cassius constantly gives him advice that, although often immoral, makes good tactical sense.
That last trait (Brutus has much worse judgment than Cassius but keeps overruling him) continues in the last two acts. Otherwise, the play just loses steam. The rival sides fight a civil war and also quarrel among themselves. The conspirators lose and kill themselves. The plot loses its focus, and the characters become less focused, as well. Antony, whatever his faults, seemed deeply and genuinely devoted to Caesar. He won over the crowd, in part, by showing that Caesar left his fortune to The People. He then starts meeting with his allies, Octavius and Lepidus and trying to figure out how to avoid actually having to pay, to say nothing of drawing up hit lists and bartering over who is to be killed as calmly as over a sack of flour. Meanwhile, Brutus and Cassius get into a quarrel with all the emotion maturity of a couple of toddlers, then kiss and make up and decide they need a drink. And Brutus makes yet another decision so stupid that even his enemies can't believe he was that dumb. And Cassius keeps going along with it, out of devotion that overrides common sense. I suppose you could call this character development. Mark Antony is a cold-blooded cynic who was apparently just putting on an act before. The noble Brutus is not above childish tantrums. And Cassius is not, after all, Iago, but has a real devotion to the man he so cynically manipulated in the first act. (They are brothers-in-law, and address each other as brother). But as anyone who reads me knows, I make a sharp distinction between character development and character violation. What Shakespeare does to his characters in the second half of the play borders on character violation, especially for Cassius. In the early part of the play, Cassius is shown as intensely resentful of Caesar being top dog. That is his real motive for the assassination, plain, pure and simple. So why is he so submissive to Brutus as top dog, even when Brutus proves time and again that he is completely unqualified for the job? I think it would be possible to give Cassius better character development, somehow, but Shakespeare signally fails.
*We had a sort of debate what is the difference between these two types of arrogance.