CASCA
O, he sits high in all the people's hearts,
And that which would appear offence in us,
His countenance, like richest alchemy,
Will change to virtue and to worthiness.
CASSIUS
Him and his worth and our great need of him
You have right well conceited. (1.3.162-167)
Earlier we saw Cassius try to flatter his friend Brutus into believing that he would make a better Roman leader than Caesar (1.2). Now it seems pretty obvious that Cassius was trying to manipulate his pal, because here he acknowledges that the conspirators want Brutus on their side. He's popular with the commoners and will make the plotters against Caesar look "virtu[ous]" rather than "offen[sive]."
Quote 5
CASCA
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.
[...]
Marry, before he fell down, when he perceived
the common herd was glad he refused the crown,
he plucked me ope his doublet and offered them his
throat to cut. An I had been a man of any occupation,
if I would not have taken him at a word, I would I
might go to hell among the rogues. And so
he fell. When he came to himself again, he said, if he
had done or said any thing amiss, he desired their
worships to think it was his infirmity. (1.2.269-272; 274-282)
Here Casca describes Caesar's theatrical behavior in front of the adoring crowd. After refusing Antony's offer of the crown three times, Caesar faints dramatically, and the crowd loves him all the more for it. Casca suggests that when Caesar appears before his followers, he presents himself as an actor of politics, and the "tag-rag [common, or poor] people" respond to his theatrics like an enthusiastic audience at a playhouse.
Quote 6
CASCA
And then he 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)
In the last passage, we pointed out how Casca knows that Caesar's dramatic refusal of the crown and fainting spell are just cheap tricks used to curry favor with the "hoot[ing]" and "clap[ing]" crowd. Here Casca continues to describe Caesar's adoring crowd as though they were an audience watching a performance in an Elizabethan playhouse.
In this passage, Shakespeare also seems to be making an inside joke when Casca refers to the loud audience's "stinking breath." Crowded Elizabethan theaters were notoriously smelly places (there being no mouthwash or deodorant at the time). Plus, Elizabethans thought the plague was contracted by breathing in strong odors like bad breath. So when Casca says he was afraid to laugh at Caesar and the crowd because he didn't want to open his "lips" and breath in the "bad air," he's suggesting that 1) the crowd's bad breath might make him faint like Caesar and 2) he might catch the plague. So basically, Casca is bagging on Caesar's rowdy crowd and Shakespeare is bagging on the theatergoers who pay to watch his plays at the same time.