Sienna Miller ''really, really hoped'' 'American Woman' would ''fall apart'' before filming began.

The 37-year-old actress - who has Marlowe, seven, with former partner Tom Sturridge - plays Deb, a 31-year-old grandmother whose daughter goes missing in the film and she admitted the idea of losing her child gave her ''nightmares'' so she was hesitant about the project because she didn't want to be reminded of her fears.

She said: ''I was really, really hoping the movie would fall apart before.

''I was just like, 'Please, please, I don't want to do it!'

''It's something that you consider if you're a parent. It's strangely available to you, the imagining of that.

''And with those things that terrify me, or feel like nightmares - those are the things I tend to be drawn to.''

Despite her reservations, Sienna loved ''every minute'' of the ''intense'' shoot.

She told Total Film magazine: ''It was really intense, because I was in every scene.

''But I really thrive in that kind of environment, it turns out.

''I really liked being in control of a character, and not having to make the most of something relatively insignificant and top hope that it's impactful in the end - which is ultimately harder work than having the space to map something out in its entirety.

''It was six-day weeks and no money for anyone. It was a labour of love, but it felt so dense as an experience that I loved every minute of it, however exhausting it was.''

The film was shot two years ago but the character is still occupies Sienna's thoughts.

She said: ''I sort of fell in love with that woman. I just wanted to hold her close. I still think about Deb. Where is she now? What is she doing?

''It was the most fulfilled I've ever felt creatively in a film, for sure, by a long way.''