The words spoken by Caesar at this point are:
Et tu, Brute? - Then fall Caesar!(III.i.77)
The Latin line, 'et tu, Brute?' means, 'And you, Brutus?' This conveys Caesar's sense of shock at seeing Brutus among his killers. After this, it seems, he simply gives up. We gain the impression from Shakespeare's dramatic rendering of the scene, that it is this mental blow of seeing a close friend turned traitor that causes his final collapse, rather than the physical stab wounds.
The dramatic and poignant effect of the moment of Caesar's realisation is heightened by Shakespeare choosing to give the line in Latin - which of course, is the language that the real Caesar and the other Romans would have spoken. It is the personal friendship between Caesar and Brutus that gives rise to the central conflict, the overwhelming moral issue of the play: Brutus's fight with himself over the killing of Caesar, which he comes to accept as a poltical necessity to prevent the possibility of Caesar's establishing lifelong one-man rule.
Caesar does not appear friendly with any of the other conspirators, and displays animosity towards Cassius particularly, just as Cassius despises and envies him. Just before his assassination he appears quite haughty and grand with the whole lot of them as they petition him, while referring to himself in god-like terms:
Hence! Wilt thou lift up Olympus? (III.i.74)
However, he is soon brought literally crashing to the earth by the conspirators, and most of all, by the anguish on seeing Brutus among their number.