Elisabeth Moss calls Jeremy Piven's exit from Speed-the-Plow due to mercury poisoning "highly unprofessional." Credit: Slaven Vlasic/Getty Images

Don't hold back, Elisabeth Moss! During "Plead the Fifth" on Andy Cohen's Watch What Happens Live Thursday, May 23, the Mad Men actress sounded off on her 2008 Speed-the-Plow costar Jeremy Piven.

"What, besides eating too much sushi, caused Jeremy Piven to quit Speed-the-Plow?" Cohen asked the actress. (Piven, 47, famously left the Broadway production early, claiming he had mercury poisoning.)

"That's a very good one. I could go on and on," Moss, 30, said, with a big smile on her face. "Being highly unprofessional," she gave as her answer.

"We were all surprised," Moss added of Piven's exit. "It came out of nowhere. He just didn't come back one day."

But she did run into him not long after he left. "I saw him like a month later at the Golden Globes," she shared. "When he was supposed to be really sick."

Cohen also asked Moss about the rumors that her Mad Men costar, Jon Hamm, has "impressive anatomy." "Is it true what they say about Jon Hamm's ham?" he asked.

"Well, I mean, we've all seen the pictures so, I mean, I would suppose so," Moss said with a smirk. "That's all I know."