The two often appear in the same scene. And in one Season 1 episode, the two actors were in the same bathtub.
"This series is an incredibly fun one to film," Radcliffe said. "Jon is a very funny guy, and sort of very irreverent and … we have a lot of fun, particularly doing the bath scene.
"As soon as we both got in it, we were like, 'Oh man, this photo is going to go everywhere!' "
It was an odd mix of comedy and dark drama.
"The funny thing about that scene, actually, was that it wasn't a particularly funny scene," Radcliffe said. "It was very much a scene where there's … sort of a huge plot point in that episode is being established, and it's actually quite tragic."
As a result, the writers "were very much just going, 'We need to do something to make it funnier. We'll put them in a bath.' "
It's a long way from Harry Potter. "The most exciting way of doing my job, I think, is to try and do as many different styles," he said. "Not just styles of film, but of TV shows and theater."
Although it's not as much about distancing himself from Harry as some seem to think. Radcliffe said there's "a part of me [that] is connected to Potter, but maybe not the way that everyone thinks. Where everyone kind of just thinks, 'Oh, he's doing all this to put that behind him and get away from it.' " Which is not the case.
Although author J.K. Rowling just released a short story about Harry and the other characters in their mid-30s, Radcliffe said his "inclination is to say no" to another go-round as Harry.
"It's not even a hypothetical at the moment" because Rowling's story "is a very short piece that I'm not sure is, of itself, worthy of adaptation to film."
"He's at least, sort of, 12 years older than I am now. So I don't think I'll have to worry about that for a long time."
As for Radcliffe and Hamm playing the same character despite the 10-inch difference in their height, the actors don't worry about it. And neither do most of the show's fans.
"If they get hung up on that, then there's going to be plenty of other logical problems that they are going to find with us, I'm sure," Radcliffe said.
Scott D. Pierce covers television for The Salt Lake Tribune. Email him at firstname.lastname@example.org; follow him on Twitter @ScottDPierce.