"It's actually the culmination of my life so far," said the Harry Potter star Daniel Radcliffe last month. "It's everything I have been living for."
And no, he wasn't talking about the recent release of the most grown-up instalment of the Harry Potter saga thus far. Radcliffe was explaining his excitement about featuring in an episode of The Simpsons.
Radcliffe, of course, is the latest in a long line of celebrities who have appeared in the world's favourite animated sitcom. In fact, there have been so many cameo appearances like Radcliffe's - from Buzz Aldrin to Tony Blair, Paul McCartney to the White Stripes - that The Simpsons holds the world record for the most guest stars in a television series.
But what made Tony Blair - the prime minister of the UK at the time, let's not forget - want to expose himself to potential ridicule? After all, he appeared to come out of the 2003 episode rather well… until Homer says, right at the end: "Wow, I can't believe we met Mr Bean."
The official line Blair gave at the time was that he wanted to promote Britain as a tourist destination post-September 11 (the Simpsons clan were on a trip to the UK). But if that were really the case, he could have gone on an American chat show.
The real reason is obvious. The Simpsons was - and remains - cool. The writing is sharp and irreverent. By appearing on it, guest stars associate themselves with everything cutting-edge and engender a sense that they're not afraid to laugh at themselves; that, incredibly, they're human after all.
Take Olivia Newton John's appearance in the spectacularly successful musical melodrama Glee last year. The former Grease star re-made her Physical video with the show's conniving gym teacher Sue Sylvester, and revelled in portraying a darker side of herself. "It's a mixture of mean and diva," she said to People magazine at the time. Of course, it won't have harmed her bank balance when the Glee version of Physical was a hit on iTunes.
Glee went on to feature cameos from Britney Spears, Gwyneth Paltrow and Josh Groban, and, in the end, began to lose some of its charm in doing so. At least Paltrow was playing a character; as soon as Groban appeared as himself, it all felt a bit contrived. But the queue to appear on Glee is still long. And that's because the show itself is not only popular, but in its best moments (of which there are admittedly fewer) it's still great fun.
And surely this is the reason Ted Danson regularly appeared on Larry David's Curb Your Enthusiasm, as a version of himself. The improvised script no doubt stretched Danson as a comic actor, but he was also among friends - as he once said on collider.com: "You have fun, you all know each other, and you really don't care." It was less like work and more like a hobby.
So, happily, money isn't always the driving force behind cameo appearances. Sure, Justin Bieber probably had one eye on developing his career when he signed up for CSI - and the producers would certainly have been aware that, at the moment, Bieber is a major draw.
But when Sir Ian McKellen appeared on Coronation Street in 2005, there was no motive beyond being on a show that he loved. Crucially, his respect for the soap meant he didn't dominate the scenes in which he appeared.
It was a similar story when Johnny Cash quietly played a murderer-cum-country musician in a 1974 episode of Columbo. Indeed, if ever you happen across the detective, it puts paid to the idea that celebrity cameos are a 21st-century phenomenon. Billy Connolly, Leonard Nimoy, Faye Dunaway and William Shatner have all been found out by the man in the mac.
But back to Harry Potter. It won't have escaped anyone's attention that the films are essentially a roll-call of the great and good of British cinema. In the current instalment, the likes of Alan Rickman, Bill Nighy, Michael Gambon and John Hurt have the most minor roles.
But, as Jason Isaacs - whose Malfoy character has figured large in the past but probably has less than five minutes' screen time in this particular film - explained to the BBC recently: "There is as much love and care invested in Harry Potter from the people making it as there is in the people watching it."
In the end, the knowledge that he's part of something special is enough for Isaacs to forget about how few lines he has. Which is probably the best compliment a film can be paid.
