Jessica Brown Findlay says she was "naive" to flash her chest in one of her early roles.
The 23-year-old, who found fame as Lady Sybil in ITV period drama Downton Abbey, played a teenager who has an affair with an older man in 2011 British film Albatross, her big screen debut. In one scene, she lifts her top to show her bare chest and says "Is that good enough?" when her character is asked to prove her age to buy cigarettes.
Jessica told the Radio Times: "To be honest, Albatross was naivety and not knowing that I could say no. I had no idea what was going to happen and thought I was going to be shot from behind."
The star also sheds her clothes in Labyrinth, the forthcoming Channel 4 adaptation of Kate Mosse's best-selling novel.
But Jessica, who quit Downton despite the show's huge success, said she wouldn't do Labyrinth's steamy scenes again.
"In Labyrinth it is a very fleeting moment that we shot in one take, but I did find it very odd being naked in front of lots of people and I think it's awful that women get so criticised about their bodies," she said. "I think if you're going to do a nude scene, be honest and natural. Otherwise I'd be starving myself for ever, which I just couldn't do!
"The idea that actresses would work out at the gym for a thousand hours beforehand... I was drinking pints and eating burgers. But actually, it's not something I would do again."
The actress, who has just filmed Winter's Tale opposite Will Smith and Russell Crowe, said her ambition was to write while owning "a little tea and sandwich shop". "Hollywood is not for me. I love acting, but I also love London," she said.
Jessica, whose character died after giving birth in the last series of Downton, left the show because she did not want to fall into her "comfort zone".
"My contract was ending and I was unsure about signing away another year. Leaving terrified me, and that's what made me want to do it," she said. "Being afraid and going into the unknown excites me, and what scared me more was to keep going and then one day discover it was all I could do and wish I had pushed myself more. I'd prefer to fail and fall flat on my face."