I think there's also a model agency scam.
I think there was a TV or radio feature some time last year - young would-be models, mostly young women, were being enticed to pay money to participate in a model agency that then didn't find them work, and didn't need to try very hard to do that because they were making money just on registering members. It may still be going.
Snopes.com has covered a few times what they consider to be various fake-employment scams, including various "anonymous helpers/money mules in fraud" deals. They say you will NEVER have to pay a sincere employer to hire you. I'm not sure if that's exactly true - for some jobs you probably require protective clothing and you may have to buy your own - but it's a good reminder to stop and think about what you're getting into. And if you don't mind getting into fraud, consider that you'll probably be dealing with people who have spent more time thinking about their scheme than you have, are smarter than you, and don't mind putting their hand into your pocket without you noticing, as well. It's unlikely to be a long-term association.
Anyway, yeah, extras... non-speaking roles, background artistes who are supposed just to not do anything attention-getting... you could be disappointed with your part.
On the other hand, there have been ventures where people who just really want a particular film project to be made have donated money and been allowed to appear in a crowd scene, but I don't think that applies to "Swedish Dentist 6".