"I remember what the phrase was which helped sell it a while after I found out it was Doctor Who," he says. "They said: 'It's an ensemble piece, Brad. You're not going to be the Doctor.' I went, 'well that's good, because I don't understand any gobbledegook'. I said, 'Who is going to be the Doctor then?' And they went 'We don't know yet, we haven't chosen them.' I went, 'Oh, OK, fine. OK, cool.'
"A week later – who's the Doctor? 'Dunno, we haven't chosen them yet.' And that went on and on and on. And I said 'How can I gauge what I'm going to do with the part?'And they said 'You're gonna be like an older companion. And it's gonna be a bit like the bridge on the Enterprise. It's gonna be an ensemble piece. It's gonna be like...' And I went 'I'm in!' as soon as they said that, because I'm a massive Trekkie. I went 'I'm in, I'm in, that's it, OK done it'."
The original Enterprise was staffed by various characters who performed assigned roles, with William Shatner's Captain Kirk taking the lead above them all. It's possible that the new series will see a similar dynamic, with Whittaker's Doctor taking a step back so Tardis team Yasmin Khan (Mandip Gill), Ryan Sinclair (Tosin Cole) and of course Graham (Walsh) can share enough screen time.
Walsh has previously expressed misgivings about taking on the role before he knew who the Doctor would be, but says he was assured by Chibnall that the series would be "ground-breaking", and so he decided to take a "leap of faith".
When Walsh did eventually find out who the Thirteenth Doctor was, it was in the same way as everyone else did: by watching the reveal after the 2017 Wimbledon final between Roger Federer and Marin Cilic.
"I found out [about Jodie] with the rest of the world," he says. "I got a phone call in the morning – 'make sure you're watching the tennis'. I sat watching the tennis. And I'd watched Broadchurch, with Jodie in it, and I think Jodie's brilliant. And I went 'Oh my god it's a girl! It's female! Fantastic, I love it!'
"And now, the lightbulb... it went 'Now I get it!' They were going 'Oh we're gonna change it, it's gonna be this, it's gonna be different... it's gonna be seen as groundbreaking. I went 'I get it, I totally get it.'"
Doctor Who series 11 will air on BBC1 on 7th October
