>>24503872

He's talking about the wit of William the person, Jonson is remembering "when he said". Its clear he is talking about William's conversation he is not talking about writing in the context of the passage:
>His wit was in his own power; would the rule of it had been so too. Many times he fell into those things, could not escape laughter, as when he said in the person of Cæsar, one speaking to him...