Sienna revealed to Britain's New magazine: "Edie was unhealthily skinny. But it's quite funny that Andy was skinnier in the film. Guy was taking the p**s out of me a lot! He'd say 'Little fatty Edie!' He was so skinny!"
The 25-year-old actress also revealed the director tried hiding her food in a bid to get her to lose weight for the part.
She said: "The awful thing was Guy was really disciplined about losing the weight and I wasn't. I didn't want to.
"The director used to steal my bagels but he gave up because I love my food.
"I'm an energetic person but I don't work out and I do eat a lot. So Andy was actually skinnier than Edie in the film, which is not how it was."

In other news, Sienna Miller insists her sex scene with actor Hayden Christensen in ‘Factory Girl’ is fake.
The actress stressed that only porn stars made movies where the sex was for real.

"I'm not gonna assume that everyone's been on a film set but it's not like you're doing a sex scene and the camera just finds you two - there's a room of 20 people and it's really embarrassing," the Sun quoted her, as telling New magazine.

"There's a name for people who actually do have sex on film and get paid for it, but I'm not that kind of actor and neither is Hayden," she said.

However, the actress admits that she is ‘flattered’ that people think the scene is real, for it shows that she and her co-star did a ‘good enough job’.

"So the sex scene is not real but I'm flattered that people think we did a good enough job that it might be," Miller said.