What do wild parties, college lectures, massive armies, and zombie hordes have in common? If you're a filmmaker, they all require a ton of extras.
But not everyone has a big enough budget or network to acquire them, which can be pretty stressful considering that at some point in your filmmaking journey, you're going to want/need to shoot a scene that requires a crowd. That's why it's crucial for every indie filmmaker to learn how to trick your audience into thinking that your five roommates (god help you) are actually a throng of wasted co-eds thrashing to EDM -- or whatever your scene calls for.
This video from our buddies at Film Riot offers some techniques on how to do this both in-camera with clever blocking and camera settings, as well as in post by compositing multiple shots of the same extras.
Creating crowds, armies, and hordes of people in post is quite common, since the logistics and cost of hiring, managing, and directing hundreds, even thousands of people can make even the most hardened filmmaker weep. If you know your way around After Effects (or something similar), compositing is relatively straightforward. (If you don't know but want to learn, this lesson on how to "populate mass groups" from Tuts+ is excellent and very thorough.)
That technique is great if you want to show your crowd in an extreme wide shot, but you're eventually going to have to get in close. That's when you bust out the in-camera blocking and camera techniques. Let's run through a few quickly:
Group your extras close together
Because groups group -- and closely. Do whatever you need to do to make the frame look populated.
Shoot with a long lens
This not only creates depth, but it gives the illusion that there are more people than there actually are. The bokeh also helps to take the focus (literally and figuratively) off of the extras and put it on your subject.
Place your subjects on a different plane of field than your extras
Again -- for depth, but you can also get a nice parallax effect in the back and foreground, adding to the illusion that there are tons of people busily walking through the scene (because it's so crowded).
Get close-ups of each of your extras
Cut-ins of individuals in a crowd -- another little bit of trickery that tells your audience that there are lots of folks. It's also helpful, because the backs of people's heads aren't easily identified, so you can use them over and over again as you get your close-ups of different extras. (Just make sure to change your location within the room, as well as your blocking.)
Granted, these techniques may not work (or will at least be more challenging) for certain scenes you want to shoot. Replicating extras for a scene with a zombie horde is going to be considerably easier than doing the same for a scene inside a crowded lecture hall, which is why you'll have to use your best judgement when choosing how to shoot scenes with only few extras that would otherwise require many.
However, you can rest assured knowing that your project isn't going to derail because you can't procure enough extras. After all, cinema, at its core, is one big, fat lie; it's an illusion. Everything is about perception, so whatever you have to do to convince your audience that what they're seeing is in fact 50 extras and not just your five roommates -- do it.
Believe me, they'll want to be convinced of that, too.