Gatiss - who's written this Saturday's episode of the show, 'The Empress of Mars' - told Digital Spy that he has "a mixture" of feelings about the sweeping changes coming next year.
"Peter's been magnificent," he said. "I love him this year, particularly - I think he's just completely in his groove. I think the season is excellent - really refreshing and fun - and I think he's just on fire, really.
"So it's a great way to go, but obviously I'll be very sad to see him leave. I think he's been amazing... and I can't believe how quickly it's gone. It just seems like the blink of an eye since the last time.
"But the show depends on change and it's exciting as well – not just the new Doctor but a whole new approach and a new team [with Chris Chibnall replacing Steven Moffat as showrunner from 2018].
"It's always exciting and a little frightening, but also thrilling because there's all that to look forward to. So I can't wait to see who [the new Doctor] is - but I have no idea who it is!"
Gatiss wasn't sure if Capaldi would be leaving Doctor Who when he wrote 'Empress' – now his final script for the actor, and possibly his own final contribution to a series he's been working on for 12 years.
"I suspected that Peter would want to go with Steven because it just felt like time, and I think that does happen. Something that Doctor Who fans, I think, rarely take in as part of the equation is the human element - as the Daleks might say! The fact that there are lots of human emotions in the mix here.
"Part of you, of course, would love to see Peter under Chris Chibnall. Y'know, David [Tennant] nearly stayed, and David under Steven for a year would've been such a different beast, it's fascinating to think.
"But, those are the arguments that Doctor Who fans have in the pub, not the ones that actors have when they feel like it's time to go."
Via Digital Spy
