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.
Cassius does not seem sincere, but a great conspirator and jealous over the popularity of Caesar. As he stands against him and stands Brutus against him by telling fabricated story to him. At the end of scene ii Act I he plots to turn Brutus against Caesar by throwing forged letter. Through this letter he proves that men distrust him. Besides it will convince Brutus that Caesar will be tyrant after getting power. Cassius aims to take advantage of Brutus’s weakest point.
Cassius goes deeper into his opposition, as Casca says that Cassius has lost his humanity and has become beast. He succeeds in his plan because Brutus, after reading the letter, accuses himself of sleeping while Rome is threatened; he plans to kill his friend, Caesar to save the nation. At last the conspirators kill him.
Cassius is indeed a talented general after the murder of Caesar he insists on Brutus to kill Antony, a great danger and don’t let him bring the body to the forum and speak funeral oration.
Cassius is indeed played the role of inhumanity and hatched plot of killing of a great sympathetic to the poor:
When that the poor have cried,
Caesar hath wept.
Caesar bequeathed a sum of money from his personal holdings to every man in Rome. The words of Caesar about Cassius were appropriate and true.
He should be sincere to Brutus, but due sincerity is not found. He accuses Brutus of having wronged him because he eliminated his man in crime of taking bribes. So he was not sincere but a conspirator.