The A Field In England star will play Patrick Troughton - the second Doctor - in An Adventure in Space and Time, penned by Mark Gatiss.
"I was honoured to be part of it," Shearsmith told DS. "Mark's... had that idea for a long time and he's finally got to do it this year. It's the perfect time for it to happen, with it being the 50th anniversary in November.
"I think he just always had this idea that I would [play Troughton]. It's very small - I'm literally only in a very small little bit, but it was lovely to be part of it."
Shearsmith also described David Bradley's performance as first Doctor actor William Hartnell as "absolutely brilliant" and called Adventure "a lovingly rendered thing".
"Mark was in his absolute element, wandering around the TARDIS… the attention to detail was immaculate," he said. "He's done a really good rendering of that time and that story. I think it's a real treat."
The 43-year-old added that he would "love" to follow up his Troughton role with an appearance on Doctor Who itself, but ruled himself out as a potential replacement for departing lead Matt Smith.
"Oh God, no! I've literally not even thought about it," he said."I've had a few people tweeting me that, saying, 'You should be the Doctor'… I'm like, 'What??'
"It could be literally anyone. I can't imagine who we're gonna get. But it would be good to go older, I think, because they've done two younger ones."
An Adventure in Space and Time will air on BBC Two later this year.
Reece Shearsmith's new film A Field in England will be the first ever UK film released simultaneously in cinemas, on DVD, on Film4 and on VoD on Friday July 5. www.afieldinengland.com
> Reece Shearsmith on A Field in England: "It becomes very strange"
Watch Digital Spy's Geek TV special on Matt Smith's exit and the 12th Doctor below: