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 night-caps
and uttered such a deal of stinking breath because
Caesar refused the crown that it had almost choked
Caesar; for he swounded 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. (1.2.9)
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 theatre, I am no true man. (1.2.11)
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.