One day, you might be pretending to pilot a helicopter; the next, doffing your cap to Downton's Lady Edith. And the chances are that the next day, you'll be pacing up and down wondering where the next job's coming from.
Being an extra - or background artist, to use the official term - is an unreliable way to make a living, but is certainly never boring. And you don't need to be particularly beautiful, or even have any acting experience. An unusual appearance can sometimes be an advantage - being tattooed or obese may be just what's wanted, and pensioners can often get more work than students.
What you do need, say casting agencies, is stamina, the ability to follow orders and a readiness to be available at short notice - sometimes, just a few hours. It helps a great deal if you have particular skills such as riding, fencing or even pole dancing.
"The sort of people we look for have very good availability, in and around London in particular. Ideally, they'll have their own transport - there's often no public transport and a call for six o'clock in the morning," says Tony Gerrard, general manager of Ray Knight Casting.
Agencies tend to be very selective about who they take on - many are deluged with would-be stars. It's vital to follow all instructions about, say, emailing rather than phoning and submitting photos that conform to requirements. And if they say they're not taking anybody on right now, they mean it: hassling them won't get you anywhere.
But if something specific comes up, says Gerrard, agencies will place an alert on their website. "Because of Mr Selfridge, for example, suddenly girls with long hair of a certain height were in demand - and with no tan," he says. "And there was an old people's home for Derek, with Ricky Gervais, and they wanted really old people in their 70s and 80s."
It pays to be cautious about which agencies you apply to, with many charging upfront fees to register artistes - but then making no effort to find them work. Media and entertainment trade union BECTU describes this as the biggest problem facing the industry.
Gerrard agrees. "A lot of agencies make their money by just signing people up. We only make money if someone gets work," he says. He warns against open casting sessions at hotels, where aspiring actors pay over the odds for photo shoots, and agencies that are willing to take people on on the basis of a written application alone.
Taking part in a long-running show can lead to regular work - if you're lucky enough to get cast as a background market trader in Eastenders, for example. But it can also shut you out of future work: if you've been a nurse in episode one, you can't be a bus driver three weeks later.
For all these reasons, very few people make a living as an extra - most just regard it as a part-time income. One who's made it is Ian Bellman, who's appeared in everything from Inspector Morse to Tomb Raiders.
His first job came after attending an open casting for Shadowlands - a casting that had actually closed by the time he arrived.
"I stood at the bottom of the stairs, and there was a big queue and a sign saying 'sorry, this is the end of the queue - no-one will be seen after this point'," he says. "I just moved it."
It was a steep learning curve, says Bellman, but he's been in work ever since. It's a lot harder for new entrants, though, he warns. "A lot of the agents just keep you hanging around. You get pencilled in but not booked, but if you're not available then you never get work from them again," he says.
Unless you are lucky enough to be given dialogue, or asked to be a stand-in, the pay is pretty poor. BECTU lists a basic daily rate of £84, or £105 for a nine-hour night. Overtime is £8 to £10 per half hour, and there's a bit more for changing clothes, getting rained on or driving a car, for example. However, not all jobs pay this much - and when you count in the time waiting around, the travel and so forth, it's no way to get rich.
But that's not really the point, says Bellman.
"It's the memories, the crazy stuff. Sometimes, it seems like a dream, because it can be exciting and a lot of fun. Firing cannons, riding beautiful horses or being on a set with Natalie Portman," he says.
"The oddest was in The Worst Week of My Life, when I had to play a dogger. They dressed me up in chaps and a leather posing pouch and put me in a car park with a Page Three model. Then I realised it was actually a real police station and the police were standing around watching the filming."