But when they meet up with another couple - Carlos (Eduardo Noriega) and Abby (Kate Mara) – everyone but Roy notices that Carlos seems to be putting the moves on Jessie. That dredges up memories of Jessie's wild child past, and kicks off an increasingly dangerous flirtation that leads to twists and turns that you absolutely will not see coming.

The charming Mortimer, best known for her roles in Lovely & Amazing, Notting Hill, Lars and the Real Girl, The Pink Panther and a guest stint on 30 Rock, talked to me earlier this week about this delicious thriller, her enthusiasm for being an action heroine, and working with Martin Scorsese and the "brilliant" Tina Fey …  

Hey, Emily. Thanks so much for taking the time to talk to me today. So, I watched Transsiberian last night, and I have to tell you, I was up for several hours just thinking about it. It's such a great thriller!

(Laughing) I'm sorry I ruined your night!

No, not at all. It was worth giving up a few hours of sleep to be that entertained …

I know, I think it is … I'm proud of the movie. I feel like it asks difficult questions of the audience and it doesn't attempt to answer them, which is what I like. And it has a sense of humor. I think you're left feeling kind of unsettled.  Which is cool.

It's cool that the whole story is being driven by a female character, too …

Yeah, I think it's great that you see such a complex character, and that she's female. Very often, I've found that characters have been simplified, so yes, for it to be a woman and be such a baffling character, I think, is rare and a great treat to play.

Is it true that you only signed on for the role two days before filming began?

Yes, and I had no time to think, which, in some ways, I think was the best preparation, because I just had to feel instinctively what the character would be like. I understood this character, even though what she does is very questionable and strange and not something I've ever done. If I had had more time to prepare, I might have worried too much about whether or not she was coming across as sympathetic and trying to explain her, in a way that might have damaged the movie.  All I could do was show up and do the scenes and try to be as real as I could in each moment, but not try to argue my case as the character.

So much of the storyline unfolding the way it does depends on Jessie and her husband Roy meeting these strangers, Carlos and Abby, and putting their trust in them right away. Did jumping into the movie on short notice also help establish that? Not really having the time to think about that aspect of your character …

Yes, and it's interesting that you should say that. I think that that's true, and I think that's very similar to the feeling of doing a film in general, even if you do have preparation time.  A huge amount of trust is involved, because you show up and you just have to jump in. You've got to get to know people who are strangers extremely well, extremely quickly and a kind of intensity and familiarity builds up which is deceptive.  You know, you don't really know these people at all, but you feel in some way that you've know them all your life. And I think that's exactly the predicament this character finds herself in on that train with that young couple. It allows her to access sides of herself that were dormant, that she tried to suppress for a long time, her dark side. In meeting these, especially Carlos, there's a tragic inevitability about that side of her being released again.

Do you enjoy working on this kind of movie, with all the action, physical scenes?

It was fantastic.  I felt more than I have ever felt like I was definitely in a movie, you know? I was funning through snow-covered fields, with bare feet and men chasing me with guns, and hanging off the back of trains and generally being a part of the kinds of shots I've only seen in real movies. There's something amazing about being one of those girls, and it was really cool. Though, as they told me, I'm not actually that convincing when I'm running.  (Director) Brad (Anderson) tells me that he had to really edit the hell out of the scene with Jessie running away in the snow, because I look so bad running.  I just don't look cool, and in fact, my friends will make me run for the bus just to give themselves a cheap laugh. So, I really do have to be edited quite a bit in order to be a convincing action heroine.

Well, it is very convincing. Especially in the scene where you're carrying that big stick to fend off, um, without spoiling anything, someone who's chasing you while you're running through the deep snow … (WARNING! Spoiler ahead! SPOILER AHEAD! Skip this answer if you don't want to have a major plot point SPOILED!)

(Laughing) That was easy to do.  For some reason, bashing someone over the head until they died … I found something inside myself that knew exactly how to do that. But it was exhausting.  Killing someone with a stick is really tiring. For about four days, I had aches and pains and bruises. It really takes it out of you murdering someone.

Great release for stress and any pent-up aggression, though, huh?

Yeah, it's amazing, she's just bashing away. I wasn't really bashing the real guy, of course, but I was bashing this dummy next to the camera, over and over and over and over again. Really kind of letting it rip, and it was bizarre. And yeah, kind of thrilling.

You have another thriller coming up, Shutter Island, with Martin Scorsese. Have you enjoyed the experience of working with him?

It was incredible, and I would do it again in a heartbeat. He's the most amazing guy, especially for somebody who is such a brilliant man and so revered and has such a master of his arts, and then he's actually incredibly accessible and nice and easy to talk to. You know, a lot of these people are hard to talk to because you're sort of scared of them, but he enjoys talking to people, and he likes talking about movies, so it's a treat just being around him and listening to him. You don't feel like an idiot when you ask him a question, because he's happy to answer it.

You gave such a memorable performance as Jack's fiancé Phoebe on 30 Rock … any chance of returning to the show, or doing other American TV?

I'd love to. I loved doing TV, I love that show specifically, and I'm sure that every experience on TV isn't the same as that one, but I love the speed of it. There's something so fascinating about it because it's so quick.  And there's not enough time for the powers to be to assess whether or not what you are doing is suitable.  It feels less deliberated over than a movie does, which is so much, nowadays, sort of decided by committee. (TV) feels really exciting and the 30 Rock cast is amazing. Tina Fey is phenomenal, and I can't really understand how she can be so brilliant at what she does and so nice and so not annoying. And Alec Baldwin was a great treat too. So I had a great time, and I'd love to come back, but I was such an objectionable character … I don't know quite in what capacity I'd return.

That's true, though, given how wacky all the characters are on that show, Phoebe could probably come back and fit right in.

Haha, well, now, after this movie, I could come back and bash everybody over the head with a stick.