Along with co-star Karen Gillan (Amy Pond), Arthur will leave the Doctor Who in the fifth episode of the current series 'The Angels Take Manhattan'.
Darvill had the following to say in an interview with Blastr:
"Me and Karen made a decision to leave when we did, but we're definitely going to miss it. It does feel like the right time to move on."
Darvill discussed Doctor who as a programme that is about change:
"The worst thing you can do is outstay your welcome on something like this. We've had such a good run and such good stories, but the whole programme is about change. It's about things changing and evolving. So it's sad to leave, but it feels like the right thing to do."
Darvill originally expected to only appear on Doctor Who for "three or four episodes." He called the length of his time on the show "really surprising".
He also addressed his need for healthy and varied career as an actor:
"As an actor, I wanted to do different things, and, for me anyway, I'm quite an impatient person," he explained. "I want variety so signing onto something for so long was a risky thing to do. It's so easy to get bored."
And touched on why he stayed as long as he has:
"But with the scripts that were thrown at us there wasn't a chance of that ever happening, 'cause it wasn't just turning up to work to do the same thing every day. It was something new every single day. Something new to explore about the character."