The 26-year-old actress - who plays Dylan Thomas' wife Caitlin MacNamara in the 'The Edge of Love' - was unaware her co-star Keira Knightley was using fake nicotine sticks for the scenes in their new movie.

She said: "It was the 40s and everyone smoked then."

When Keira interrupted and said, "They were fake", Sienna replied, "Were they? Mine weren't!"

Sienna added to BANG Showbiz: "In the 40s it was wartime and everyone was living on the edge and it looks beautiful, its part of the director John Maybury's world."

Sienna - who reportedly recently split from Welsh boyfriend Rhys Ifans - also revealed she was so taken with West Wales, where they filmed a lot of the movie, she is considering buying a property there.

She said: "I would really love to have a place in West Wales. It is such a beautiful area and I enjoyed spending time there."