I'd heard great things about Session 9. What's more, I can even see why. It's one of those movies where something bad is going to happen, but you don't have a clue what it might be. Will it be supernatural? Will it be psychological? Or will it be trying to have its cake and eat it, like The Turn of the Screw? That's quite an interesting area and I'd enjoy discussing it, but that wouldn't be a nice thing to do in a review. It takes a long time for this movie to tip its hand. At least there's no big mystery about where the danger lies when you're watching, say, Texas Chainsaw Massacre or Night of the Living Dead. This is more like The Blair Witch Project in that it will give a lot of people the screaming heeby-jeebies through keeping everything unsaid and not actually showing us very much. Well, up to a point. You'll get most of your answers eventually.

Our heroes are an asbestos removal company who've been brought in to clean up an abandoned asylum. That sounds good already, by the way. It's a refreshing set-up for a (potential) haunted house story. These aren't reclusive millionaires, psychic investigators or dumb teenagers, but working men doing a job. Only gradually do we get to know the cast, whose personalities are themselves part of the film's puzzle. They're a livelier and more fucked-up crew than they look at first.

1. There's Hank, who's stolen Phil's girlfriend and is a bit of a dick. He doesn't know when to shut up.

2. There's Gordon, the boss. He's a calm man, but he's also wound up really tight by a new baby daughter, some kind of domestic problem and financial troubles with the company. If he hadn't won this contract, he'd have gone bust. There's also something wrong with his leg. He's just taken on...

3. ...his mullet-headed nephew, who despite being an idiot teenager is a decent guy.

4. I thought Phil was being played by William H. Macy until I saw the credits. It's David Caruso, who seems to have built quite a career out of working on US TV cop shows. Hill Street Blues, NYPD Blue, CSI: Miami...

5. Mike's the clever one with a legal background who should be in a better job than this. He's the one who knows icky stuff about the asylum's background and gets interested in researching its past. He's played by Stephen Gevedon, who co-wrote the film's screenplay with the director.

The other important person in this movie is called Mary Hobbes. She's mad. She's also dead. However it's possible to listen to tape recordings of her therapy sessions, which is where the movie's title comes in. Session 9. I've always liked the trick of being kept waiting for something to appear, simply because it's the title and so you know it must therefore be significant. They do it here. Incidentally the film was going to include a homeless woman living in the asylum and spying on everyone. She played a big part in the original ending, but apparently this confused test audiences who assumed she had to be Mary Hobbes. Out she went. Apparently you can find these deleted scenes on the DVD and they help flesh out the narrative, although I haven't watched them.

One thing I like about this film is that it gets seriously creepy despite a completely down-to-earth tone. It's not afraid of letting its characters be in a good mood. You hear voices and even see things, but the film's tone is so realistic and straightforward that you almost ignore it. There can't be anything supernatural going on, can there? You know how it is when you're watching something that's so obviously a horror movie that you abandon any kind of reality and just watch as an exercise in the rules of the genre? This is not such a film. It's realistic, it's low-key and people react as they do in real life. When someone disappears, it's not a movie moment. You won't know what's going on, or even at one point whether or not you'll have just seen someone die.

I'm going to try to talk about the ending. This is going to be delicate and I'll try not to spoil anything, but suffice to say that there's the straightforward reading and then the one suggested by the film's last line. They both work. I still don't see what's going on with the rocking chair, but it's creepy as hell and I suppose that'll have to do. Apparently they shot this movie in a real abandoned mental hospital, almost all of which was unsafe for the film crew to go into. The director got the idea of using it from driving past it every day.

Stephen King talks about sizzle and steak in a horror movie. Sizzle is where you're keeping the audience on tenterhooks. Steak is where the monster jumps out and eats someone. This is basically a sizzle movie, although I will give it credit for finding something new to do to someone in a horror movie. That was nasty. Ewww. Don't think it doesn't have some steak prepared too. What it does particularly well is to keep its characters so real that you're genuinely absorbed in their story rather than just waiting for the good stuff, which makes certain later scenes all the more unsettling. They're not charming people. They're not even particularly colourful, being asbestos cleaners instead of character traits on legs. However for the space of these 100 minutes, they'll turn you into an asbestos cleaner too.