In Act 1 scene 2, Brutus is dejected. This seems like the perfect time for Cassius to plants the seeds of conspiracy. Cassius points out two things: Brutus seems sad; the people respect and honor Brutus. Brutus then share that he is afraid that the people will make Caesar king. Cassius agrees and then he says that Caesar is not fit for such glory. For example, he has a "falling sickness."
In the next scene, Cassius divulges his plans for conspiracy, as the omens that have been reported to him confirm that Caesar ought not reign. In this way, he persuades more people to be a part of the conspiracy.
In Act 2, we see Cassius' words to Brutus have changed him. Brutus comes to the conclusion that Caesar must be killed, even though Brutus still loves Caesar. But Brutus's love for the Republic is stronger.
In Act 3, Cassius is afraid that the plot has been discovered. So, he urges the conspirators to work quickly to get the job finished.
Cassius drives the conspiracy, but he has a way to make people own it as well.