If I were Brutus now and he were Cassius, / He should not humour me. Cassius means that if he were in the place of Brutus, with the respect of everyone, including Caesar, he wouldn't let a person such as himself lead him away from that place of honor. He would act in his own self-interest and stay on the good side of Caesar. But Cassius considers Caesar his enemy, so he needs to draw Brutus into a conspiracy against Caesar. And he can do it, because Brutus is noble and will sacrifice his self-interest for the general good.