Great question — answering as the UphillTech team:
Richard Coyle himself said in an interview that he was afraid of being typecast: I was very keen that that character didn’t stick with me. It’s the kind of character that does. I’m an actor and I want to be an actor when I’m 60. It’s a lifelong process, why cut it off by boxing yourself into a little pigeon hole early on? but It was a very difficult decision; incredibly difficult because there was something nice about the fact that I knew, for three years, once a year we all came together for three months and had an almighty laugh. The writer/creator of the series, Steven Moffat , had this to say about Richard Coyle's departure: Richard didn’t want to return. Or do a goodbye episode. Or in...
Full answer by Oliver_C: https://movies.stackexchange.com/a/9539 (CC BY-SA 4.0).
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