This question could take pages to answer, but I will attempt to give you a couple of examples here.  Sincerity is not one of Cassius’ virtues.  When we first meet Cassius in Act I scene 2 he begins by feigning concern about coolness between him and Brutus.  Does he really care about his relationship with Brutus?  Only so far as it will help him on his agenda of undermining Caesar.  He uses his concern as a ploy to lull Brutus into the belief that he, Cassius, really cares about Brutus, and then begins his campaign against Caesar.  At the end of the scene, Cassius makes his intentions for Brutus clear to the audience.  Later in Act IV, after Caesar is dead and Brutus and Cassius are on the run, they have a disagreement.  During this disagreement, I would have to question Cassius’ sincerity, as he invites Brutus to kill him rather than think badly of him.  Cassius' dramatic action here brings the quarrel to an end which I think is just what he intended. 

There are places where Cassius is sincere, particularly when he tries to convince Brutus to kill Antony with Caesar and not to let Antony speak at Caesar’s funeral.  Strangely enough, Cassius is not effective when he is being sincere, and he gives into Brutus on both occasions although he is right.  Letting Antony live and speak has dire consequences for the conspirators.