In the US, it is common practice to write spec episodes of existing television shows in order to convince producers that you should be let in to a writers' room. The system works like this: if you want to write for, say, House, then you write a spec episode of Battlestar Galactica, or Desperate Housewives or anything else that ISN'T House. The producers of that show want to see that you can write but they don't want to read your take on Hugh Laurie mainly because of legal reasons. If they don't like your script but then use a storyline that was similar to your episode, then you'll get all huffy and want to sue. Writing spec episodes is a big deal for new writers in the US, and a lot of time and care goes into the process as it may lead to a big break.
Over here, writing a spec episode doesn't hold the same currency at all. If you write a spec Dr Who script and send it to Casualty, they won't care how good the writing is because the script isn't applicable to them, and, more crucially, you're an 'unknown quantity' as a writer. If you write a spec Dr Who script and send it to Russell T Davies, well, he won't read it, because you might sue if he uses something similar to your episode. Of course, you could charm and hustle Russell to read your Dr Who script and convince him to take you on. It has been known to happen (a writer being hired based on the spec ep they wrote for the producer's show), there's always an exception, but generally, in the UK, if you want to get ahead, you have to write an original spec script. Your own idea, characters and story.
Producers and execs in this country want to know what your 'original voice' is like. But even if they're impressed by your work, a commission may still elude you because the next thing they want to know is if you're been through the system, and if you're able to cope. In other words, they prefer it if a writer has experience and has been through the mill, i.e. the process of going through 1st draft, notes, 2nd draft, all the way to production. That's why they'll be more inclined to go with someone who's had a credit or two on Doctors, or has done a notable radio play, or theatre piece.
This might sound like it's pretty pointless to write a spec episode of your favourite show. Well, it's not. It all depends on what you want to get out of it. Regardless of what a producer may think in terms of actually hiring you, a spec script will always be a writing sample, and if it's fairly decent, then that's always going to leave a good impression. And, on some occasions, you may be asked directly to write a spec episode of the show that you're trying out for, like EastEnders, or Not Going Out.
When I was fresh off working on the set of Black Books (1st series, god, almost 9 years ago!), I decided to write a spec episode, naively thinking it would be snapped up by the producer for the second series. The producer liked it, complimenting me on getting the tone and characters right (when she had seen many spec scripts that hadn't, apparently) but they were going to use two experienced writers for the 2nd series, and my episode wasn't going to get a look in. Still, the script has served me well as a general sample, especially if a producer, agent or exec wants to read a good batch of scripts from your portfolio. I've put the script up on my site (in the blog download section), in case you want to have a peek. Looking back, it's a bit on the short side and patchy in places but it's not bad, all things considered.
So, if you're just starting out, it may be helpful, and indeed fun, to write a spec episode of your favourite show. Let your excitement and imagination run free, and get your portfolio underway before moving on to your own personal stuff, where your 'original voice' can shine.
To end on a more festive note: if you're not really bovvered by the Hallelujah hoopla that's been going on, here are a few other tracks that might spice up your Christmas playlist: James Brown - "Santa Claus Go Straight to the Ghetto", Stevie Wonder - "One Little Christmas Tree", The Beach Boys - "Little Saint Nick".
HAPPY CHRISTMAS! Don't eat too much, now.