Image caption Pearl Mackie was revealed as new Doctor Who companion at the weekend

Peter Capaldi has revealed how a theatre trip to see the new Doctor Who companion turned into a test of his acting skills.

The actor went to see The Curious Incident of the Dog in the Night-Time in early April - before Pearl Mackie was announced as his new co-star.

When he was invited to meet the cast after the show, Capaldi and Mackie had to pretend they had never met.

In reality they'd already filmed a top secret scene together with the Daleks.

"I wanted to see the show but I didn't expect to be asked backstage to meet everyone," Capaldi told the BBC at this week's launch of a Doctor Who artwork exhibition at London's Cartoon Museum.

Image copyright Twitter Image caption A picture posted on Twitter on 7 April shows Capaldi posing with the cast of the West End hit at the Gielgud Theatre - with Mackie on the far right.

"I knew Pearl had the job, but I couldn't tell anyone and she couldn't tell anyone," he said.

"It was only because I had worked in that theatre previously and all the front of house staff knew me, so they said 'Peter, you must go backstage'.

"So, that was a great performance of mine pretending that I didn't know Pearl!"

Mackie's role as new Doctor Who companion Bill was revealed on BBC One on Saturday, during the FA Cup semi-final in a special scene showing Capaldi's Time Lord and Bill being chased by Daleks.

The 28-year-old actress replaces Jenna Coleman, whose character Clara Oswald left the show in 2015.

Filming for the 10th series of the long-running science fiction show starts next month and will air in 2017.

Image copyright Paul Vought Image caption Peter Capaldi was a guest at the Doctor Who artwork exhibition launch at the Cartoon Museum

Speaking about his new co-star, Capaldi said: "The nice thing is that she is not part of the Doctor Who world - so the whole idea of travelling in time and space will be new to her.

"It's a while since we've had a companion like that."

'Level-headed'

Doctor Who executive producer Steven Moffat, who was also at the exhibition launch on Thursday, said he was confident Mackie would cope with her new-found fame.

"There is nothing you can say to prepare people for being suddenly terribly famous," he said. "She is a very level-headed, clever girl. She'll be absolutely fine."

He added Mackie had been the obvious choice at the auditions.

"We saw some fantastic actresses for that part, they were all amazing genuinely all amazing - it's not often you can say that. But she was the first one through the door and she was just extraordinary."

Media playback is unsupported on your device Media caption The trailer announcing Pearl's arrival aired on BBC One during half time of the FA Cup semi-final

And how difficult had it been keeping the casting secret?

"No one got it until some insider dealer went and placed a bet," Moffat said.

"It wasn't me - I wouldn't do that - but I'm sure we'll find out who it was."

Doctor Who: The Target Books Artwork Exhibition is at the Cartoon Museum 28 April - 15 May.