Doctor Who season 12 introduced Jo Martin as a "new" Doctor - and she had no idea she was playing the part. Doctor Who showrunner Chris Chibnall had promised season 12 would redefine the series, and he wasn't exaggerating. The season finale rewrote Doctor Who's history, revealing the Doctor is no Time Lord - she is the Timeless Child, originating from another dimension or universe, and with unlimited regenerations.
The season also introduced actress Jo Martin (Birds of a Feather, The Brittas Empire, and The Bill) as a never-before-seen incarnation of the Doctor. Her character was introduced as a tour guide who lived in Gloucester, Ruth Clayton, but that was revealed to be a false identity. This mystery Doctor had used Time Lord technology to hide in the present day, replacing her own consciousness with a fake one. The revelation left viewers staggered.
Speaking in Doctor Who Magazine #549, Martin has revealed she was just as surprised. She's an old-school fan of Doctor Who herself, growing up watching the adventures of Tom Baker and Peter Davison, and her son is hooked to the modern series. She actually auditioned for the part of Ruth, described as a tour guide. She began to realize something was wrong when they tested her against scenes where she was piloting the TARDIS. "Sometimes with auditions, they don't give you the actual text," she noted. "They'll give you something different just to see how you read. I thought maybe that's what it was. So I put it to the back of my mind, went into the audition and hoped for the best... I had absolutely no idea I was going to be a Doctor!"
Martin only learned the truth when Chibnall gave her a phone call to tell her she'd got the part - and revealed she was playing the Doctor. "I wanted to run down the street going, 'Guess what? Guess what? GUESS WHAT?' But I had to button it and be like, 'I'm just playing a tour guide." Martin couldn't even tell her son, simply because she knew he was too much of a fan and would be too excited not to tell his friends at school. He only learned the truth when he was watching the episode.
Martin wasn't the only one taken aback to learn Doctor Who season 12 was going to introduce a new Doctor. The April issue of Doctor Who Magazine also includes an interview with costume designer Ray Holman, who designed the Doctor's distinctive - and superb - new costume. He deliberately waited until Martin was cast for the part before deciding on the new Doctor's look, and wound up incorporating Kente cloth in order to represent her. This is an African wax-print fabric, and both he and Martin were thrilled when they found the beautiful, vibrant patterns. The new Doctor's costume has earned almost universal praise from the typically divided Doctor Who fanbase, and Martin, too, seems to have been a hit.
Source: Doctor Who Magazine #549