Why is it important that Cassius gets Brutus to join the conspiracy in Shakespeare's Julius Caeser?
Cassius knows that he is not well liked, whereas Brutus is liked and respected by everybody. According to Plutarch's Life of Brutus, which was one of Shakespeare's chief sources of information for Julius Caesar:
Cassius had the reputation of being an able soldier, but harsh in his anger, and with an authority largely based on fear...
Cassius displays his violent temper in the quarrel with Brutus in Act IV, Scene 2, when Brutus tells him he is "much condemned to have an itching palm."
I, an itching palm?
You know that you are Brutus that speaks this,
Or, by the gods, this speech were else your last.
Cassius' dialogue in this scene is full of such threats and real or affected anger, showing that he habitually uses anger and threats to get what he wants. He is also a miser. Shakespeare uses the plain-spoken Casca to illustrate how people who know Cassius regard him. When Cassius invites Casca to dinner (so that he can try to recruit him for his conspiracy against Caesar), Casca replies:
Ay, if I be alive, and your mind hold, and your dinner worth the eating. Act I, Scene 2
Casca has known Cassius all his life and knows what kind of dinner he can expect from Cassius. It will include the worst wines and small portions of everything. And Casca knows he wouldn't be invited to dinner if Cassius didn't have some ulterior motive.
Cassius knows he is unpopular because of his selfishness, greed, miserliness, and violent temper. That is why he needs Brutus to serve as the putative leader of the conspiracy. Cassius would like to be the real leader but to use Brutus as a figurehead. He thinks the kindly Brutus is malleable, but he finds out he was wrong. Once Brutus has persuaded himself to take part in the assassination, he quickly becomes the de facto leader. Cassius is dismayed and frustrated, but there is nothing he can do because most of the conspirators like Brutus much better, and most of have only joined because they took Brutus to be the leader and the instigator.
It is of the utmost importance to Cassius to get Brutus to join his plot against Caesar. Without Brutus it might be impossible to recruit anyone else, but with Brutus committed to the assassination, Cassius has no trouble persuading a large number of other important Romans to join them.
In addition to the important reasons given by previous answers, we may also observe that there is more than a hint that the scheming Cassius would rather like to bring Brutus down to his own level. Cassius's soliloquy at the end of Act I, scene iii is absolutely crucial here, part of which is quoted below:
Well, Brutus, thou art noble: yet, I see,
Thy honourable metal may be wrought
From that it is disposed, Therefore it is meet
That noble minds keep ever with their likes:
For who so firm that cannot be seduced?
Caesar doth bear me hard; but he loves Brutus.
Cassius appears to be playing the part of the scheming villain to perfection, here. He remarks that Brutus is 'noble' but can still be influenced to actions less than 'honourable'. He says that worthy men like Brutus should only keep company with like-minded people because no-one is 'firm' enough to resist all temptation. He implies, therefore, that, by associating with himself (Cassius) Brutus can be swayed from his noble high ground.
We have seen, in this scene, that Cassius has been plying Brutus with a variety of reasons as to why Caesar should be killed, and that Brutus does have misgivings about Caesar's political position which Cassius does his utmost to exploit. When left alone, Cassius appears envious not just of Caesar but also of Brutus's great friendship with Caesar. We sense that, setting aside all other factors, it is important for Cassius to get Brutus to join the conspiracy simply because he wants to wreck the personal friendship between Brutus and Caesar.
This is an important question. There are two main reasons why it is important for Cassius to get Brutus to join the conspiracy.
First, if Cassius can get Brutus to join the conspiracy, then it would be much better for public relations. In other words, the people respect Brutus and trust him. Therefore, if Brutus is marked with the conspirators, the people will believe that Caesar was aiming at kingship and that the assassination was necessary.
Second, Brutus has a veritable heritage as his great ancestor of the same name was part of a conspiracy to rid Rome of its first kings. This family connection would bring greater legitimacy to the cause of the conspirators.
simply because the commons consider Brutus as a very honourable man accordingly they do love and respect him so when brutus say that Caesar was a proud man and that he deserved his death they'll believe him. Brutus for cassuis is like the rich alchemy that turns every cheap and mean thing into gold