Can you explain the theme of friendship in The Tragedy of Julius Caesar?
It's interesting that you'd see a theme of friendship in the story. I would say most of the so-called friendships are more of loosely-knit alliances of convenience. Brutus is not truly a friend of Cassius, even though Cassius professes his friendship to him. Likewise, Cassius is simply using Brutus to add pull to his growing conspiracy; he feels people would trust Brutus' explanation of why Caesar had to die, rather than his own or that of one of the other conspirators.
On the other side of the fight, you have Mark Antony, who is a so-called friend of Caesar's. He comes "not to praise him, but to bury him" in his spirited requiem, yet proves he did love the man. However, he is no true friend of Ocatavius, the heir to the throne. Instead, Octavius, Caesar's nephew, has a power struggle with Mark Antony to determine who should truly take command out of the three men fighting the conspiracy.
There are few friendships, and even fewer people anyone can trust in this tragedy. However, it's an interesting theme, and can be investigated through a careful observation of the interactions of Caesar and Mark Antony.