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Peter Capaldi obviously had to keep secret the fact that Pearl Mackie was going to be the next Doctor Who companion.
But that proved tricky when Capaldi ended up backstage at Mackie's show The Curious Incident of the Dog in the Night Time - and they had to pretend they didn't know each other.
"I wanted to see the show, but I didn't expect to be asked backstage to meet everyone," he told BBC News. "I knew Pearl had the job, but I couldn't tell anyone and she couldn't tell anyone.
"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!"
Capaldi also talked up Mackie's character Bill, saying that she'll be a refreshing change from recent companions.
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"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," he said.
"It's a while since we've had a companion like that."
The reveal of Mackie as the new companion has proved immensely popular, with the 'Friend from the Future' video attracting more than 2.7 million views on Facebook. It is also the most shared content in Doctor Who history.
Watch it again below:
