Randy Credico, a comedian and left-wing political activist, has an appointment on Sept. 7. With Robert S. Mueller III. Before a grand jury. Under oath.

And he is planning to do impressions, because that’s what he does.

“It’s like, ask me if I’m going to breathe? Of course I’m going to do impressions,” Mr. Credico, 64, said. “I’m taking the grand jury very seriously but doing impressions is part of the package just to calm my nerves,” he said, adding, “You got to give that grand jury some comic relief.”

Mr. Mueller is interested in Mr. Credico’s odd friendship with Roger J. Stone Jr., the self-proclaimed political dirty trickster, Republican consultant and longtime political adviser to President Trump.

Mr. Credico is seen in New York political circles as a dedicated if madcap activist. But he acknowledges that he also took part over the years in some of Mr. Stone’s acts of political deception.