John Slattery, who plays ad executive Roger Sterling, and Elisabeth Moss, who portrays copywriter Peggy Olson, dreamed up a prank they said didn't come off.
"We were going to do this thing when you just mouth the words with no volume, no sound, coming out, and it wasn't even that funny," Slattery told People magazine. "Elisabeth [Moss] would say that I chickened out, but I think I was just trying to concentrate on what I was doing."
Vincent Kartheiser, who plays young executive Pete Campbell on the AMC series, also tried to pull a trick on Hamm.
"That didn't work either," Slattery says. "Hamm is too smart. He kind of is un-Punkable."
Hamm said he was just concentrating on his directing of the April 1 episode that will follow Sunday's season premiere.
"It was not my choice to come in and go, 'I'm gonna do my thing. Here's a whole different attitude!'" Hamm said. "My job was to not drive that train off the tracks, but keep it going as smoothly as I could.
"I think I did an ok job," he said. "We'll see."