Speaking to Doctor Who Magazine, Gibson revealed she found out she got the part in slightly surreal circumstances: "I went to get a spray tan for the NTAs [National Television Awards], and I was in the room waiting to dry off when I got this WhatsApp call, asking me to join the call with all my agents."
She continued: "I picked up the phone and they were like, ‘Are you good at keeping secrets?’ I said, ‘Yeah.’ They said, ‘Are you really good at keeping secrets? Cos you’ve got it.'”
Gibson was later announced as the newest Doctor Who companion during Children in Need.
At the time, she said: "It is a gift of a role, and a dream come true, and I will do everything to try and fill the boots the fellow companions have travelled in before me.
"And what better way to do that than being by the fabulous Ncuti Gatwa’s side, I just can’t wait to get started."
Showrunner Russell T Davies also said: "It’s the great honour of my job to find the next generation of talent, and Millie shines like a star already. She’s brilliant, dynamic, clever and a wonderful actor. As a Coronation Street fan, I’ve seen Millie survive chases, guns and sieges, but that’s nothing compared to what lies ahead for Ruby Sunday."
The 60th anniversary will also feature appearances from Neil Patrick Harris and Bernard Cribbins.
Watch below: Billie Piper reacts to Russell T Davies returning to Doctor Who.