The identity of the “Doctor Who” companion is one of the closest kept secrets in the industry — akin to who dies on “The Walking Dead.”

Pearl Mackie won the coveted role of Bill, the new sidekick for Series 10, debuting Saturday, April 15 at 9 pm on BBC America. But the secrecy surrounding the casting was nothing like anything the actress had experienced before.

Mackie was performing in a play on the West End when her agent recommended her for a role in “Mean Town” — an anagram of “Woman Ten” — the female companion for the tenth series.

“There’s no way I’m ever going to get that,” Mackie recalled to Variety. “It’ll just be a nice jolly outing for me to audition.”

But after reading the script, she felt an affinity for the role.

“I felt like I know this girl,” Mackie explained. “I felt like she was a real rounded human being I related to. I still didn’t think in a million years I’d get it, but I thought I’d give it a good shot.”

The first audition was just a standard read to camera. Two discouraging weeks went by without a response. “If you ask any actor, if you don’t hear anything after a week, you figure you don’t have the part,” Mackie explained.

But then she got a recall, and discovered her next audition would be opposite the Doctor himself, Peter Capaldi.

“I was very nervous and I tried not to let it show,” Mackie admitted. While reading for the scene where Bill and the Doctor first enter the TARDIS, Capaldi made an unusual request.

“‘How about standing up and acting it out?'” Mackie recalled her costar asking. “He was running back and forth pressing buttons and pulling imaginary levers and I was just standing there with my mouth open thinking that this is the maddest thing that has ever happened to me. I don’t know what is going on here or what I’m doing.”

That real-life disbelief helped Mackie land the part.

“Turns out Bill is meant to be thinking that in the scene, otherwise we might not be sitting here having this conversation,” she joked.

Mackie also remembers the first time she walked upon the actual TADIS set. “All the buttons actually do stuff! They all move and the turn on and off. It was absolutely incredible.”

While Mackie kept mum about plot details, she did gush about her experience working with Capaldi, especially after learning this would be his final run as the Doctor.

“I just feel so grateful that I was able to work with him. He’s so creative to work with. He’s so generous on set and in rehearsals and generous with his own experiences, which helps with some of the abstract situations we find ourselves in on ‘Dr. Who.'”

Mackie also felt relief after the casting news was finally released to the public.

“I had kept it secret for two months from getting the job to it getting announced,” she revealed. “It was mad. I avoided all of my friends because I was terrible at lying. I thought the best thing to do was to fall off the radar. My friends were messaging me and asking ‘what are you doing?’ And I was like ‘this play I’ve been doing for nine months has suddenly needed more attention in recent weeks.'”

Mackie gathered her friends for a champagne celebration when the role was announced. But it was the reaction of a stranger she remembers the most.

“A mom tweeted that her little girl was mixed race and also had an afro. When the trailer came on, the girl said ‘Hey mom, she looks like me.’ If can help young children recognize [someone like] themselves on prime-time, then that’s amazing.”

Mackie makes her debut as Bill on “Doctor Who” this Saturday on BBC America. Get a sneak peek at Series 10 below.