Incendiary director, Sharon Maguire did a Q&A at Sundance Film Festival. She had this to say about casting Matthew Macfadyen:
With Matthew, that was a surprise piece of casting even for me. He was suggested to me and I thought, no, I had only ever seen him as Mr. Darcy in Pride and Prejudice and I thought he was far too young, handsome, and thin to play that role. But he really wanted to play the role and he wanted to be a character actor rather than play a leading man type role. So he came to see me and said, “I think I can do this.” And I believed he could.
You can read the full interview here