I can’t speak for anyone else, but I’m going to spend the rest of the week sending several anonymous postcards to Hollywood power-players. Written on them will be the most severe warning I can possibly imagine: Sex and the City 2.

Why would I embark on such a madcap endeavour? Because, all of a sudden, a number of very sensible television showrunners have decided to make films based on their shows. HBO is officially moving forward with a long-rumoured Deadwood film. The departure of Rick Grimes from The Walking Dead was immediately followed by an announcement that Rick Grimes would be starring in his very own Walking Dead spin-off movie. And now, most troublingly of all, producer and showrunner Vince Gilligan has announced that he’s currently working on a two-hour film “set in the existing Breaking Bad franchise”. Neither Aaron Paul or Bryan Cranston are signed up – at this point – but rumours that it will be focused on “a kidnapped man and his quest for freedom”, point to a possible return for Paul’s Jesse Pinkman.

Now, some caveats. These might all potentially turn out to be very good, very worthy pieces of art. By all accounts Deadwood was cancelled too soon, so a film might make for a satisfying full stop to a story that was left in limbo. The Walking Dead is such a heavy genre piece that, at the very least, The New Adventures of Rick Grimes might turn out to be a passable zombie flick. And God knows by now we should all know better than to ever doubt Vince Gilligan.

But still, don’t. Please don’t. It never works out when a movie is made of a TV show. Quite apart from Sex and the City 2 – which single-handedly destroyed what had until then been a sly satire of a certain type of New Yorker – let’s also remind ourselves of Baywatch, Miami Vice, CHiPs, Dark Shadows, the Flintstones films, the Bewitched film, The A-Team, The Saint, Lost in Space, Charlie’s Angels: Full Throttle and Land of the Lost. All of them, without question, were cynical attempts to piggyback on our knowledge of an existing franchise. And all of them were legitimately depressing.

Facebook Twitter Pinterest A Deadwood spin off is in the works Photograph: Allstar/Cinetext/HBO

And these are all classic small-screen TV shows, made when there was a notable prestige gap between film and television. Back then, television was film’s poor cousin; brainless, repetitive muck churned out of an idiot box in the corner of the room to help sell detergent.

But that isn’t the case any more. For the last 15 years or so, television has become a true writer’s medium. All the golden age shows you ever fell in love with – The Sopranos, The Wire, The West Wing, 24 – came about because writers understood the shortcomings of film as a vehicle for long-term narrative. Series have been compared to novels for such a long time that the observation has grown a little trite, but it still holds a kernel of truth.

Facebook Twitter Pinterest Cooking up a surprise? Bryan Cranston and Aaron Paul in Breaking Bad. Photograph: AMC/Everett/Rex

Take Breaking Bad, for instance. It’s a great show because it’s a show. It took time to put us in a specific place at a specific time, and went to huge lengths to make us understand why Walter White had reacted to his diagnosis the way he did. Remember how slowly it moved? It took 50 episodes, broadcast over four and a half years, just to get us from White’s 50th birthday to his 51st. This deliberate pace let us linger over details that would never even been considered if it had been a film. It let the story wander off down weird little avenues. Fly – the bottle episode widely considered to be a series highlight – would have been scrubbed from the timeline completely. Breaking Bad the show was rich and satisfying. Breaking Bad the film might have been giddy and violent, but it would have been entirely superficial.

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Which isn’t meant to do cinema down, of course. Film is its own specific thing, and it’d be stupid to say that all TV shows are better than all films. But why, once you’ve had the freedom to tell an entire sweep of a multi-part story over several years, would you ever want to try and force all that into something as slight as a film?

Again, only an idiot would second-guess Vince Gilligan. He is one of the best writers we have, and there isn’t a chance on Earth that he would have committed to a Breaking Bad film unless film was the best possible way to tell that specific story. I will happily let him disabuse me of my ignorance. But, until then, I have postcards to write.