Peter Capaldi has revealed how a trip to see Pearl Mackie in The Curious Incident Of The Dog In The Night-Time before she was announced as Doctor Who's new companion tested him.
After being invited to meet the cast, Pearl and the 58-year-old had to pretend they had never met although they'd already filmed a Dalek scene together for the long-running BBC One series.
"I wanted to see the show but I didn't expect to be asked backstage to meet everyone," Peter told the BBC. "I knew Pearl had the job, but I couldn't tell anyone and she couldn't tell anyone."
He added: "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!"
Outgoing Doctor Who executive producer Steven Moffat said Londoner Pearl is "just extraordinary".
Pearl, who will play Bill, was announced as the new companion at half-time during the FA Cup semi-final between Manchester United and Everton.
She said: "I'm incredibly excited to be joining the Doctor Who family. It's such an extraordinary British institution, I couldn't be prouder to call the Tardis my home.
"Peter Capaldi is such a brilliant actor, and his Doctor is such a wacky and wonderful character, I can't wait to see what adventures are in store for him and Bill throughout time and space."
Pearl takes over from Jenna Coleman as the Doctor's assistant.
Jenna made her debut as the Time Lord's brave and resourceful companion Clara Oswald opposite Matt Smith's Doctor in 2012 episode Asylum Of The Daleks.
The Blackpool-born star went on to share the screen alongside the Scottish actor's Doctor.
Jenna bowed out after three years as the longest-serving companion since the show was rebooted in 2005.