Casca is a Roman conspirator who takes part in Caesar's assassination.
Like all the other conspirators, Casca is worried that Caesar will be crowned king, which goes against the ideals of the Roman Republic. Casca is also not a big fan of Caesar's theatrics. Check out the way Casca describes how Caesar refused the crown three times and then fainted dramatically before the adoring crowd:
And then [Antony] offered it the third time. He put it the
third time by, and still as he refused it the rabblement
hooted and clapped their chapped hands and
threw up their sweaty nightcaps and uttered such a
deal of stinking breath because Caesar refused the
crown that it had almost choked Caesar, for he
swooned and fell down at it. And for mine own part,
I durst not laugh, for fear of opening my lips and
receiving the bad air. (1.2.253-261)
Casca knows that Caesar's dramatic refusal of the crown and fainting spell are just cheap tricks to curry favor with the "hoot[ing]" and "clap[ing]" crowd. What's interesting is that Casca describes the crowd as though it were a theater audience watching a performance.
If the tag-rag people did not
clap him and hiss him, according as he pleased and
displeased them, as they use to do the players in the
theater, I am no true man. (1.2.269-272)
Casca is suggesting, by describing Caesar's "clap[ping]" and "hiss[ing]" fans, that political leaders like Caesar are nothing but actors on a very public stage. This concept isn't a new one. Shakespeare also explores the relationship between acting and politics in plays like Henry IV Part 1 and Part 2. Yet Casca's critique of Caesar and his followers seems pretty modern. His remarks could apply to just about any 21st-century politician and his or her supporters.