Why Keira's starring role is a heavy load for Sienna

But for Sienna Miller, below, the prospect of sharing the big screen with the ultra-slim, younger yet already Oscar and Bafta-nominated Keira Knightley became all the more daunting on discovering she would have to do that most horrifying of things for a rising starlet: bulk up for the part.

“I was actually pretty nervous about acting alongside Keira because my part required me to put on weight,” admits Sienna, 26, who plays Caitlin, the wife of Welsh poet Dylan Thomas (played by Matthew Rhys) forced to compete for his affections with 23-year-old Knightley, who portrays Thomas’s childhood sweetheart Vera Phillips in the forthcoming biopic The Edge Of Love.

“I just stuffed my face for the months that we were in production,” she concedes. “It wasn’t hard for me. We were staying in this incredibly eccentric Welsh manor house. I had huge fry-up breakfasts, pasties for lunch and lots of toasties.

“Then I had to face seeing myself alongside Keira, who is the most beautiful woman in the world.”

According to Sienna, Keira’s down-to-earth demeanour saved her from feeling down about herself.

“We totally bonded on the movie and it was so amazing to find a girlfriend in the business,” she tells next month’s Elle magazine, joking: “In Hollywood they don’t know what to make of us because we both drink, smoke and eat.”

Sienna’s romance with comic actor Rhys Ifans also helped to boost her confidence. Despite insisting they are not – as rumour has it – engaged, she makes it clear they are not far off.

“When his name is mentioned it makes me smile like a fool because I instantly think of him and it makes me happy,” gushes the one-time fiancée of Jude Law, who until now has refused to be drawn on her feelings for the gruff Welshman.

“We started out as friends and then it just slowly developed. I don’t think either of us were expecting it but it truly has been the most wonderful thing.”