Suetonius Caligula 54

He also zealously applied himself to the practice of several other arts of different kinds, such as fencing, charioteering, singing, and dancing. In the first of these, he practiced with the weapons used in war; and drove the chariot in circuses built in several places. He was so extremely fond of singing and dancing, that he could not refrain in the theatre from singing with the tragedians, and imitating the gestures of the actors, either by way of applause or correction. A night exhibition which he had ordered the day he was slain, was thought to be intended for no other reason, than to take the opportunity afforded by the licentiousness of the season, to make his first appearance upon the stage. Sometimes, also, he danced in the night. Summoning once to the palatium, in the second watch of the night, ¹ three men of consular rank, who feared the words of the message, he placed them on the proscenium of the stage, and then suddenly came bursting out, with a loud noise of flutes and castanets, ² dressed in a mantle and tunic reaching down to his heels. Having danced out a song, he retired. Yet he who had acquired such dexterity in other exercises, never learnt to swim.