You know it when you see it. It's a classic device to increase tension in a movie. The hero goes to see a friendly person who might have information for him. After much questioning, the friendly person denies knowing anything, so eventually the hero believes them, and leaves. As soon as the hero has gone, however, the friendly person's amiable mask suddenly drops. They pick up the phone, dial a number, and say something like "We have a problem." Or "He's on to us", or "He knows", or "He's on his way", or "Eliminate him". They might not even say anything, they might just pick up the phone and dial, looking sinister, while the music tells us that they're A Bad Guy. It's the oldest chestnut in the book (tree?), and always makes me laugh when it's used now - but it still works, and that's why it's still around.

A recent example of this is the top notch Euro-spy-shooty-bang- bang-amnesia-covert-operations-thriller The Bourne Identity. Matt Damon has just left the bank, when the strange-looking official who has been watching the whole time takes out a cell phone, speed-dials, and says "He was here." The whole point of it is to make you go oooooooh! They know he was there! And that guy's in on it! Danger, Matt Damon, danger!

The telephone. An ordinary, everyday device. Something most of us use at least once a day, without thinking about it. But the humble phone is possibly the most important (and overused) cinematic tool ever, even more so than the gun. Countless thrillers rely on the telephone to create tension. It's just so handy. You can play off the anonymity of it, with the on-screen character never knowing who is on the other end, Until It's Too Late. It's also a lifeline, one that can be threatened - if the phone lines are cut off, they feel trapped, vulnerable. It means that Something Bad is about to happen.

The earliest example I can think of, and one where the whole film hinges on the device, is Sorry, Wrong Number (1948): Barbara Stanwyck gets a crossed line and overhears someone plotting a murder. Criminally, there is no writeup for that movie yet. Phone Booth, due out in 2003, is entirely set in a phone booth - Colin Farrell hears a ringing public telephone, answers it, and a sniper tells him not to hang up, or he's dead. In the opening sequence of the first Scream movie, the psycho killer brutally murders Drew Barrymore, but drives her out of her mind with fear first - over the phone. And let's not forget, over in televisionland, the stupendous first series (season, for our Merkin chums) of 24, which couldn't have even existed without the mobile phone - it's Jack on line 2! It's the terrorists on line 6! It's Kim on line 4! Conference call us together! Get me Division! There's no answer! His phone's switched off! Dammit! And that classic scene where (avoiding spoilers) somebody tells somebody else on the phone that the somebody they're with is actually a bad somebody, but the bad somebody doesn't know that the good somebody knows! Or something.

So next time you see a thriller or action movie, keep an eye out for any scenes that just wouldn't work without this unsung hero of the movies. An urgent message is left on an answering machine warning the hero - we see the message light blinking, the killer is in the house, but the hero doesn't check the message, driving us mad with suspense. The victim tries to report the strange prowler outside, but the phone lines have been cut. The walkie talkies in Die Hard (same principle). A ringing phone making you jump out of your seat (Scorsese's superb Cape Fear remake). Tracing the phonecall of the killer, but he hangs up just before the trace is locked. The dramatic zoom in on the ringing phone, when you know it's the bad guy. Satellites tracking someone after they use a payphone. The fatal phonecall in Dial 'M' For Murder. The phone ringing in Payback, despite nobody knowing the number. Pop quiz, hotshot! Tank, get me out of here! Bish, he's lying! "I love you too, sweetheart" - no no, that's not your mother! I'm your boyfriend now, Nancy! Get me the president! How did you get this number? How did you know my name? Who is this? Hello...?