The Sin City star looked entrancing on the red carpet in a glittering red gown by British designer Jenny Packham, at the premiere of Danny Boyle's new thriller in London's Leicester Square.
Dawson's co-stars Vincent Cassel and James McAvoy both claimed they had tried hypnosis as research for the film, but it hadn't worked.
And Boyle said he didn't even try it because "directors are control freaks and I don't think they ever relax enough to be a useful subject for a hypnotist".
But Dawson, who plays a hypnotherapist in the film, said: "I think it works actually, quite well. I didn't have anything like cigarettes to quit or anything like that.
"But I remember I went in and gave her an idea of what I wanted to work on in the session. And I laid down and she put a blanket over me because when you go into a trance state it's sort of like being in between sleep and awake, so your body thinks it's falling asleep. So I got cold and then I relaxed and I went along with her voice and it was really comforting. My body did those little spasms that it does when you're falling asleep.
"And then I woke up and she said 'I know you said you wanted to work on this, but you reacted to this, this and this.' And I was like, 'How can you know all that?!' And she said my foot was kicking when she asked me about a certain thing.
"So it's actually quite interesting that your conscience will reveal itself, and if you're trained to read that, people give a lot of tells. It's almost like poker. We think we're hiding things, but we're not.
"So it was really great for me to experience that, because the premise of the film is that hypnotherapy works on such a strong level.
"So it was really necessary for me to believe that, and having been through that I gained a lot of respect for the profession, I have to say."