In Act III, scene 2, Hamlet has a brief conversation with Polonius just after everyone comes in to watch the play Hamlet has arranged. Hamlet is in a good mood and feeling witty, since he expects to find out whether Claudius is guilty of killing his father.
Hamlet: [To Polonius] My lord, you played once i' the university, you say?
Polonius: That did I, my lord, and was accounted a good actor.
Hamlet: What did you enact?
Polonius: I did enact Julius Caesar: I was killed i' the Capitol; Brutus killed me.
Hamlet: It was a brute part of him to kill so capital a calf there.
So, Polonius is the one who played Julius Caesar.