Friendship is hugely important in "Julius Caesar", not least as an oil which can be used to slick along political manipulation. Yet I'm not too sure how "real" the friendships in the play turn out to be.
Right at the start, to persuade Brutus on board, Cassius appeals to their friendship, as well as making clear how great he thinks Brutus. So, Brutus has to respond personally rather than just professionally: which gives Cassius the cue he needs:
But let not therefore my good friends be grieved—
Among which number, Cassius, be you one—
Nor construe any further my neglect
Than that poor Brutus with himself at war
Forgets the shows of love to other men.
What does Cassius say when he turns up at Brutus' orchard with some of the conspirators? He makes it very clear that the men are known to Brutus, admirers of Brutus', and Brutus' friends:
I have been up this hour, awake all night.
Know I these men that come along with you?
Yes, every man of them, and no man here
But honors you, and every one doth wish
You had but that opinion of yourself
Which every noble Roman bears of you.
This is Trebonius.
Moreover, in the famous quarrel scene in Act 4, Cassius and Brutus really argue with each other in the way that only two people who know each other can. Yet, tellingly, at the very end of the play, when Cassius names his best "friend", it isn't Brutus, but Titinius:
O, coward that I am, to live so long,
To see my best friend ta'en before my face!
There are lots of other friendship to look at: Antony and Octavius, Antony and Caesar, Cassius and Casca, Brutus and Metellus Cimber. Yet I think you'll see that most of them, like most else in this play, have a political underbelly.