You know, a movie where the idea is one that's already in culture through some kind of franchise, so there's supposedly a guaranteed audience already out there. There seem to be a lot of them around at the moment.

Possibly I'm getting old, but I'm feeling a certain cultural ennui. I'm tired of celeb culture, I'm tired of sequels, I'm tired of spin-offs, and I'm tired of over-marketing. (I'm also tired of Oxford Street, but that's just because it's horrible there.) But to a marketer, a pre-sold idea sounds great if you want to make a movie. The audience is out there! Just tap in!

I haven't seen the Transformers movie. This is partly because I gather that Michael Bay is a director who aims squarely at teenage boys, which I ain't; partly it's because, while my brother had one of those toys, I didn't get to play with it much, so I feel little nostalgia. (Mind you, I had My Little Pony toys at that time, and I don't think I'll go and see any movies based on that franchise either.) But mostly it's because I think there's an inherent problem with making movies based on pre-sold ideas, particularly in this time of Internet heyday.

The problem, artistically, is a simple one. Suppose you want to make a movie about a robot. Good movies can be made on the subject, from The Terminator to The Iron Giant. But you've got to convince your audience that a robot is an inherently interesting thing to watch a movie about. Which it isn't, necessarily. You could have a really badly-made model of a robot wheeling around a dull set speaking stupid lines, ineptly filmed, to expound a boring storyline enacted by rubbish actors. Just because one good film has been made about a subject, it doesn't mean the subject's a sure-fire winner: in art, execution is everything. Every work of art needs to convince its audience that it's interesting, and it has to do it on its own.

Now, with a movie based on a franchise, especially one that has an Internet following, you've got a problem. The people who a marketing bod can reasonably expect to come and see the film are going to come and see it because it's got their kind of robot in it. The robot doesn't have to be cool and well-designed, it just has to be from the right franchise. You don't have to work very hard get them excited about it either, because while building up to the release of the movie, they'll do that for you: they'll get on their websites and excite each other while they wait in anticipation. It's all very labour-saving.

And what you lose is the artistic objective gold standard: the sense of a virgin audience.

Movies always make certain assumptions about their audience's knowledge, beliefs and attitudes. Francis Ford Coppola doesn't begin The Godfather with a character explaining to you that the Mafia is an Sicilian organised crime syndicate with a violent history in America: he assumes you know. Elim Klimov doesn't explain that Germany invaded Belarus during the Second World War at the beginning of Come and See: it's supposed to be part of the knowledge the audience will bring with them. This is a necessary part of the process, because you can't explain everything; it would be incredibly boring if you did - and once you started, where would you stop? Do you stick at saying 'Germany invaded Belarus in World War Two and killed a quarter of its population', or do you go back through the history of Hitler and his racist theories, or do you go further back and explain that Germany is a country in Europe and Belarus a country to the west of Russia, or futher back still and start defining the concepts of 'country' and 'war'? Obviously it wouldn't work: you have to have some working assumptions about what the audience can reasonably be expected to know, like the fact that the Nazis considered the Slavs an inferior race, or that there has been a Mafia in America. You also have to have some reasonable assumptions about what an audience can be expected to feel; for example, that charismatic people are interesting, or that violence is a disturbing thing.

With a virgin audience, you are starting from scrach, working with absolute rock-bottom fundamental assumptions: good spectacle is exciting, violence is alarming, threat to the heroes is engaging. These are universal rules, and hold true in any story.

But with a pre-sold audience, you have a much bigger and much more questionable assumption: this franchise is cool. And if you assume that your franchise is cool from the outset, you have far less motivation to actually make it seem cool.

Consider, for example, the first great blockbuster, Steven Spielberg's Jaws. Prior to that movie, audiences could reasonably be expected to know that sharks were carnivorous fish - but you couldn't assume they'd know that sharks could be very big and aggressive; for all you knew, the biggest shark they'd ever seen was a stuffed dogfish at their local museum. So what does Spielberg do? He begins with that famously scary theme tune, signalling loud and clear that fear is approaching; then he shows a pretty girl run happily into the water, swim out calmly, then suddenly spend the last harrowing moments of her life screaming in agony and terror, before being dragged under the silent waters. That's pretty scary. Spielberg correctly assumes that people are frightened of unpredictable threats, and that they will find the idea of dying in pain a dreadful one. So he sets out his stall: sharks attack unseen, and they deliver horrible death. He identifies your fundamental attitudes, takes dead aim on them, and gets you really, really scared. He has to work to convince you, because he doesn't assume you're already scared of sharks.

If you want to be completely convincing, you have to start with the basic assumptions everyone will have. And cool is a particularly elusive thing to convey. It can be done, but you have to work the basics. Show Toshiro Mifune strolling into a gangster-ravaged town in Yojimbo, casually ignoring threats, then taking a relaxed walk up to the grotesque men frightening the whole town into submission, killing two of them and slicing off the arm of the third in the space of about six flashing seconds, then unconcernedly wandering back, pausing to tell the coffin-maker to start building two more coffins - or, glancing back at the injured man, possibly three - and you'll convince a pretty good proportion of the audience that this man is cool. He's shown all sorts of qualities: fearlessness, calmness under pressure, a wry sense of humour and absolute self-possession, all of which, particularly when performed by an actor with Mifune's charisma, add up to a pretty cool hero.

You can create cool. But if you start on the assumption that, to take the Yojimbo example, the idea of a samurai is inherently cool, and don't bother to convince your audience that this samurai is cool, then you can find yourself looking at a pretty lame movie. And that's an assumption that's dangerously easy to fall into if you're working with a pre-sold idea. Resting on the supposed goodwill that people will feel towards the franchise, a careless filmmaker can neglect the work of actually making whatever the film is about seem impressive, because that work has supposedly already been done. But you can't get away with that: if it's not impressive in the film, the film isn't impressive.

There's an added problem now that CGI is so widespread. You can do anything, show anything. Which means, awkwardly, that you can, well, show anything. Spielberg's shark was a mixture of underwater footage, shot at some risk to his cameramen, and a mechanical model, which broke down at intervals. The shark gave him major trouble. The result was that, out of necessity, he couldn't show it very much, and had to fall back on a traditional, splendid standby: making the audience feel as if they'd seen far more than they had by working upon their emotions. Which, incidentally, made the film an outstanding piece of audience-manipulation: it had to be, or it wouldn't have worked at all. Quality was almost a side-effect of the technical limitations. Nowadays you can CGI in a shark wherever you please; you can have one tap-dancing across the deck singing 'Nobody Knows You When You're Down And Out' if you feel inclined. So you don't have to work on the audiences emotions to make them feel as if they've seen the shark. You can just show them the shark. Which may, unfortunately, leave you with a film that contains a dancing shark and no tension, drama, timing or suspense.

Combine special effects that can take the place of skilful build-up and a pre-sold idea, and the motivation to make a movie that actually bothers to convince the audience that what they're watching is exciting has taken a serious hammering. An artistically inclined director may still decide to make a good movie, but he doesn't really have to if he doesn't feel like it.

On this logic, I decided to give the Transformers movie a miss. Perhaps unfairly; who knows? Maybe it was a masterpiece, but I decided to play the odds. My boyfriend went to see it with a mate of his, and I stayed home. I can't remember what I did, though. I'm definitely getting old.