Felicity Jones says she still hasn't got used to seeing herself on screen.
The former Archers actress, 28, has been wowing the critics in Like Crazy, a low-budget indie film.
But she said watching her performances was "horrible", adding: "It's an agonising, horrific process.
"You just analyse every bit, you're only watching yourself. So you're not really watching the film properly."
Felicity won a Special Jury Award at the Sundance Film Festival for her role as Anna, a British girl who falls in love with an American boy (Anton Yelchin) while studying in the US, in the film directed by Drake Doremus.
The actress, who voiced the role of Emma Grundy in Radio 4 drama The Archers, said: "As soon as I read the script I just thought it was so true and all the points that happen were just so honest.
"There's no gimmick or anything, it's a very unsentimental portrayal of love."
But she said playing the character was all-consuming. "There wasn't really off-camera and on-camera. Whenever we were off-camera, Drake would still be filming... for that month we were Drake's pets, we were in the characters.
"But when you get a scene and you do it and you've hopefully been truthful, it gives you an energy. And it's exciting working with people like Drake and Anton."
:: Like Crazy is in cinemas now.