Published Tuesday, 24 September 2013
The Atonement star plays the lead role in the big screen adaptation of the Irvine Welsh book, and his younger sister, Joy, has a brief cameo in which she appears opposite her elder brother in a racy sequence.
McAvoy admits he felt "weird" acting inappropriately with his sibling.
He says: "Not in my wildest dreams did I think my sister and I would be doing the same job, let alone sharing a screen, let alone me sexually harassing her.
"Thankfully, I didn't have to kiss her, or touch her or anything like that. But it was a wee bit weird. It was only hard because I couldn't stop laughing. I was the one doing the bad stuff. I was like, 'Yeuch!'"
Joy has previously said of her brother's influence: "My brother is a great role model. He encourages me to follow my dreams - it has certainly worked out for him."