What's the true measure of a television show's success these days? Ratings? Perhaps the sales of its box-sets? Or could it in fact be a series that has celebrities queuing up to make a starry cameo appearance. The news last week that Sir Paul McCartney had respectfully submitted songs to appear on the smash-hit musical series Glee is surely the final confirmation that it's crossed over into popular consciousness.
The US crime drama CSI might be the most popular series of its sort in the world, but to truly hit the headlines, this month it had to announce a future appearance from the ubiquitous teenage popstrel Justin Bieber. Hot on the heels of this revelation came reports that the French first lady, Carla Bruni, had been offered a role in the show, after expressing her admiration for it. Thankfully, Sir Paul is unlikely to be appearing in Glee in person, thumbs held aloft as the kids prance about the practice room to the Frog Chorus (although that would actually be worth watching). The show's creator, Ryan Murphy, received two CDs in the post from The Beatles legend and has promised to "do something with him" - which will probably take the form of a special themed episode. But Glee does have form in real-life celebrity cameos: Olivia Newton-John and the singer-songwriter Josh Groban both appeared as themselves in the first season. And Susan Boyle is to take on the role of a school dinner lady in the Glee Christmas special.
Newton-John's song with the gloriously mad cheerleading coach Sue Sylvester (Jane Lynch) was genuinely amusing, but the acting was merely passable. Groban's is worse. In fact, the whole notion of a celebrity cameo is fraught with danger, particularly when that celebrity plays him or herself. When real-life characters appear in straightforward high schools - as in Glee - the spell of this hitherto carefully created world is, all too easily, broken. Suddenly it's obvious that we're just watching a television series.
Still, the more successful shows can just about get away with it - just as long as the cameos are infrequent. So where Glee runs the risk of becoming a series that's more about the guest stars than the story, the cameos on one of the most successful and longest-running sitcoms, Friends, were interesting simply because they were reigned in to a few a season. Brad Pitt's appearance in 2001 as Will Colbert, a schoolfriend of Ross, Monica and Rachel's, won him an Emmy nomination for Outstanding Guest Actor in A Comedy Series. Friends also gave walk-on roles to Julia Roberts, Bruce Willis and George Clooney - all of whom entered into the spirit of it. It helped that they're actors, of course.
Cameos in these kinds of shows can work because they're inherently comic turns in, well, a comedy. But when a non-actor is parachuted into a gritty crime drama - soon to be the fate of Bieber in CSI - it's often nothing more than a ratings-grabber, human product placement. At least the country popstar Taylor Swift, who starred in CSI last year, had acted before - though MTV's review of her performance ("enthusiastic") was faint praise.
But it wasn't downright awful. One of the most hilariously bad cameos of all time is Boy George's appearance on, believe it or not, The A-Team in 1986. Completely shoehorned into a bizarre storyline about a scam involving "Cowboy George" and the obligatory armed bank robbers, any notion of the suspension of disbelief (often required when Mr T and pals were on our screens) was annihilated by acting from the so-bad-it's-good school.
The worst cameos (and by that rationale the most infamous) are always the ones that have the audience shaking their heads and asking "what is he doing there?" There is no logical reason why Chris Lowe from the British pop band The Pet Shop Boys should be driving up Ramsay Street to ask the characters of Neighbours the whereabouts of the nearest recording studio. But he did - incredibly badly. Staying in Australia, in Home and Away, the sight of Michael Palin creeping along Summer Bay's beach dressed as a surf dude who was scared of sharks had to be seen to believed.
And yet when Paris Hilton appeared on the hit US series Supernatural, she was actually rather good - because she didn't mind laughing at herself. In fact, the show's creators actually managed to use Hilton to make a social comment on the all-pervading influence of celebrity - before she had her head cut off. Supernatural's writers were clearly influenced by the show which has used celebrities better than any other series on television: The Simpsons. It's featured Mel Gibson and REM, the stars of Cheers and Tony Blair. Nearly every time, the star in question is gently made fun of - but because it's an animation it doesn't feel so forced.
Of course, Sir Paul has appeared on The Simpsons too - proving that two thumbs up from Macca is the greatest compliment of all.