The pair has teamed up for a second season of the show, playing younger and older versions of a Russian doctor at different stages of his life.
They started shooting in London earlier this summer (13), the same time the Harry Potter star was treading the boards in play The Cripple of Inishmaan, and Hamm was blown away by how hard his young colleague worked.
He tells the Los Angeles Times, "We would wrap at 5 o'clock, and he would get in a car, drive to the West End and do a two-hour show. I was, like, 'Dude, how old are you? Because when I was your age, I was not working that hard.' It's impressive. And he is impressive.
"He set a very high bar. I'm used to, when I work on 'Mad Men,' I'm used to working very hard and setting the example. And so I was very pleased that I got to work with this young actor who doesn't have to work this hard, honestly. He can take a few decades off."