Thanks to Serina in the Darcylicious Forum, we learned that One Day director Lone Scherfig had some very nice things to say about Tom in the Director's Commentary. Here is what she said:
"Tom Mison, who plays Callum, to the left, is a brilliant actor, and every moment he is actually in this film, is in the film. We have had to trim a lot of scenes down and out. But somehow every thing that Tom did, survived. He is a very interesting actor who I’m looking forward to seeing more of. Lots of nuances, and he actually manages to defend Callum, who is not that sympathetic in the script."
Needless to say, we certainly agree. If you'd like to hear it for yourself, that particular piece of commentary can be found in the wedding scene, about 1:04:00 into the movie.