What makes Alien such an effective horror film is how quiet it is. After all, most horror films make the mistake of equating fear with noise. But that's often a mistake, as silence is usually infinitely more effective. And so it is with Alien. The scariest moments are all devoid of intrusive scoring, jagged editing and overblown sound effects.

One of the best examples is when Brett is killed. He has to chase after Jones (the cat) and he's led into a dark corner of the ship. You know he's going to get killed but Ridley Scott knows that you know. Therefore the sequence is dragged out. All the time you're waiting. You're waiting for the inevitable. But Scott doesn't rush things. He racks up the tension by having Brett wander for an inordinate length of time and by having him wash his face in a shower of dripping water as chains rattle in the background. And when the Alien does appear, it's not with a sudden jolt. It just appears quietly behind Brett before killing him. And the way that the death is played on the face of the cat, which has a look of almost stunned curiosity, makes the whole sequence incredibly spooky. It's not really a crash, bang, wallop sort of horror film.

Not that there aren't any jump moments. There are actually quite a few. But because the lead-up is so quiet and ominous it makes the pay-off a whole lot more effective. For example, when Kane looks into the egg. Again the sequence takes its time, racking up the tension to breaking point with the discovery of the ship and the slow exploration of it, but when he finally has a peek and the facehugger leaps out, it has the perfect timing of a great joke.

Another fantastic piece of timing is when the Alien takes Dallas. You go through another long, drawn out sequence of him scurrying through narrow air vents before the Alien finally pops out of a dark corner. But although the moment is fantastically timed, the lead up is where the fun lies. And I have to say that the lead up is probably the most frightening thing in the film. Dallas scurries through narrow corridors searching for the Alien, but you know it's futile - we know that he won't find it and that it will find him and that it'll kill him. And the dark, claustrophobic corridors are full of iris-shaped doors that open and close slowly and forebodingly, which really does add to the air of dread that fills the sequence. And the tracker is the icing on the cake – it racks up the tension to breaking point.

The second most frightening sequence is when Ripley is finally alone in the ship. She sets the Nostromo to self-destruct so that she can take the shuttle but then with strobe lights flashing all around her she sees the Alien. She then goes back and tries to reverse the self-destruct sequence, but it's too late. It's frightening because Ripley has to run around the ship, not knowing where the Alien might be. No longer does she have the option of just hiding. And with everyone dead she no longer has anyone to talk to or help her out. She's all alone. But perversely, once everyone's dead, the film finally gets loud. There's a countdown sequence (which is brilliant in the way that it raises the stakes - not only does she have to face a terrifying monster, but she's against the clock now as well) and the ship begins to destruct. Just when Ripley might have needed the silence to find out where the Alien is, she has to run around in cacophony of noise knowing that the Alien could be anywhere.

But the silence comes back in the final sequence, and it's quite ingenious the way that the Alien is camouflaged in the shuttle. I nearly shat myself when I first saw it as a kid.

What I also like about the final sequence is that it's weirdly sexual. Ripley strips down to her underwear and then the Alien appears. And the Alien then begins almost stroking the console it's resting on. And of course, Ripley penetrates the Alien with a harpoon.

Another excellent bit of weird sexuality is when Ash the robot tries to choke Ripley with a magazine. He shoves it in her mouth and he looks rather turned on as she gags on it. Perhaps robots don't come with all the parts. Perhaps this is his way of venting his frustration. But there's also the way that the Alien kills Lambert. Its tail inches up the insides of her thigh and then you cut away and hear her screaming like she's being violated – the screaming is actually pretty chilling. And of course, Kane getting ripped open goes back to the male fear of childbirth. (The birth scene is still great. The blood, the sound and the way it takes the Alien two or three attempts to burst through is fantastic. But I do have to say that the way the Alien shoots off the table looks rather dodgy.)

Going back to the Lambert killing, though... As well as being chilling, this scene is also quite frustrating because Parker (Yaphet Kotto) gets killed as well. I mean, Parker is quite possibly the coolest man in the universe. I swear he could have taken the Alien had it not been for the cowardly Lambert. (It's quite surprising that the coward and the black character survive so long – they're usually the first two to go.)

But although I dig Parker, I think Ian Holm as Ash delivers the film's best performance. It's quite subtle the way that he hints at his true identity – I like the way he jogs on the spot to get his joints working. And he's so smug and slimy. He's more of a villain than the Alien.

But I think everything in the film is fantastic – the music, the special effects, the sequence on the alien planet, the production design, the moody atmosphere, the script. Everything works perfectly.