When a terrorist suicide bomb goes off at a London soccer stadium, a young, working-class mother is stripped of her husband and four-year-old son, and subsequently descends into a deep depression. Based on the eponymous book by Chris Cleave, Incendiary was uncannily published in Britain the day of the London bombing attacks in 2005.

The film was directed by Bridget Jones’s Diary director Sharon Maguire and stars Michelle Williams, Matthew MacFayden, and Ewan McGregor, who wasn’t there for the screening but recorded a video message for the audience. He recounted how Michelle Williams first brought him the script and he felt it was the best female role he’d ever read. He also pledged his undying support for the film saying, “To all the buyers in the audience: I will do three, four back-to-back weeks of publicity for this film. That’s a guarantee.”

Q: The script is beautiful, where did you write it and when?

Maguire: I wrote it at the top of our house while somebody else loved our child down below…. It was very much the birth of my own son that attracted me to the book. It was written by someone named Chris Cleave and I didn’t know if Chris was a man or a woman. It was written very much from a woman’s point of view so I thought, “Maybe this is another woman who’s just had a child.” So I managed to find out his email and discovered that he was a man and indeed, he had just had a child. There was definitely something “new parent” about the way the book was written, and probably the way I responded to it as well.

Q: Did you have casting in mind when you were writing it? How did you go about casting?

Maguire: I always had Ewan McGregor in mind for that character. He’s one of the few actors who can play somebody that dysfunctional and kind of likeable at the same time. As for the [the mother], I didn’t really have anybody in mind because I was so stuck on somebody I’d seen on a bus with her child and they were playing a game. Then I remembered Michelle’s performances in films I’d seen, and that she had done this brilliant British accent in a small independent film called Me Without You, so I thought she would be great.

With Matthew, that was a surprise piece of casting even for me. He was suggested to me and I thought, no, I had only ever seen him as Mr. Darcy in Pride and Prejudice and I thought he was far too young, handsome, and thin to play that role. But he really wanted to play the role and he wanted to be a character actor rather than play a leading man type role. So he came to see me and said, “I think I can do this.” And I believed he could.

Q: What about the book made you want to make it into a movie?

Maguire: I think it was the voice of this mother; her voice in the book was so compelling and also repulsive at the same time. And funny and scary. I couldn’t read the book in one go, I had to keep putting it down. I didn’t know whether I liked it or not. But I knew it had affected me in a very deep way. And while reading it I had to keep going and making sure my son was still alive in his cot. It had a very deep affect on me and I thought, “I’m going to have to do something about this.”