Kelly MacDonald claims the 'Trainspotting' cast were often drunk during filming.

The 45-year-old actress' breakthrough came when she played Diane in Danny Boyle's drugs drama alongside the likes of Ewan McGregor, Jonny Lee Miller and Robert Carlyle and she admitted she didn't understand that spending so much time in the pub on breaks wasn't anything out of the ordinary.

She told The Guardian Weekend magazine: "I thought it was OK because everyone else was [drinking]. It was like high school, you just try to fit in.

"We were just hanging out at the pub. The green room was a little caravan that we were all supposed to sit in. And the sensible people would sit, and the others would go to the local pub."

And when shooting the famous club scene where Diane met Ewan's character Renton for the first time, Kelly had been drinking all day.

She said: "By the time it came to me doing the talky bit, I was already hungover."

Although the actress is proud of the film, she admitted she has only ever seen it once.

She said: "I've not watched it since the premiere, so I have no idea. I'm sure I'm quite bad in it, actually."

Kelly had a sex scene in the movie and was thankful for how "sweet" Ewan was during the intimate filming.

She said: "Ewan had done a lot of sex scenes before that, and he was so sweet to me and talked me through it, and made me feel as comfortable as I could be.

"I know the thing now is to have people [intimacy coaches] with you and talk you through it. I've never done that. It's always been, make it up and hope it's not too embarrassing."