It was originally announced in July that the singer will star alongside Matt Smith and Michael Gambon in the festive episode.
"It was far worse than any nerves I've ever had for any [musical] performance," she told the Radio Times. "I spent a lot of time thinking about it. It's such an iconic show and it's one that we watch as a family."
Jenkins also admitted, "I'm the first to say I don't really consider myself an actress."
"I asked if I could go in and read to them, and I thought, 'Well, if I'm rubbish they won't give it to me, will they?' " she explained. "[But] I read to them and they called me on my 30th birthday and offered me the part."
The one-off Doctor Who special, titled 'A Christmas Carol', will air on BBC One in late December.