Why might he behave differently while he is with Brutus?
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Cassius is in his own way Brutus' foil. He a talented military man, and knows the rest of the play's characters quite well. Cassius resents Caesar's popularity with the people, and he is determined to be rid of him.
Brutus is loyal; he is someone you look up to; he is the epitome of honor, but he has one flaw. He is easily led, and Cassius is a master at manipulation. His whisper in Brutus' ear is constant......... and Brutus, believing in the honor and loyalty of others slowly and thoroughly buys into the plot to kill his friend and mentor.
Cassius must seem nothing but sincere when he's with Brutus. He can't allow his jealousy to be seen or Brutus will walk away from him. He's sly as a fox at hiding his true motives and making the plot seem as if it's for Rome and not for his own personal desire. Away from Brutus he can allow his real self to slip a bit.......... Cassius plays both sides, and he's adept at making everyone believe he has their own good intentions at heart.