Abisola Agbaje, 11, plays a young girl with unearthly powers in the Doctor Who episode called Fear Her.
Here she tells Newsround what it was like performing in the top sci-fi show.
Are you a big fan of Doctor Who?
I really like this series a lot. I'm really enjoying it because I think they made a really good choice with the new Doctor and the last episodes were really scary. I watch it every week, I do enjoy it.
What was it like meeting David Tennant?
He's really nice. He was always cracking jokes and he was really funny as well. When you see him on TV he has an English accent but when he speaks on the set has a really strong accent and he sounds really different. But he never got it wrong for takes!
And what was Billie Piper like?
She was funny - I liked her. She was really bubbly and down-to-earth and a really nice person.
Was it exciting being on set? Did you see the Tardis?
I went inside the really big Tardis - it was really cool. It was really really big, huge.
Chloe Webber is possessed by the Isouls. What was it like playing such a scary character?
It was weird. It was kind of strange because one minute I was Chloe Webber and the next minute I was the Isouls. It was hard because you had to change the way you spoke. On set I whispered but they put an echo on my voice for the episode.
Was it difficult doing voices and trances?
It was a bit hard to whisper because I already have a husky voice - it was the same as my voice but a bit deeper. The trance was similar - I had to roll my eyes a lot of the time and I got a bit of a headache. But I just had to go with the flow and forget my Chloe character and be an Isouls.
Did it take you long to learn your lines?
As soon as I got the script I started learning it and I kept on learning and learning. Some of the words were really hard for me to pronounce but I got used to it.
How did you get into acting?
I go to a theatre school and Andy Pryor the casting director of Doctor Who came and made everyone do an audition and I got picked. We never knew he was a casting director and when I found out I was really shocked and excited at the same time.
Have you ever starred in any other TV shows?
No - this is my first one. I want to do some more - maybe even a part in Lost!
Do you know what happens in the rest of the series?
The next episode after mine is about Rose and she is coming to the end of her part soon. I really enjoyed watching her and thought she was really good.
Will you be watching it on Saturday - if so, who with?
Everybody I know! My family, loads of my friends and I've told everyone in my theatre school to watch it too!
How will you deal with being recognised in the street?
I will be a bit star struck. When I look at the screen it doesn't look like me. I feel so normal but everyone's seen the advert now!