How to make a viral film
The short answer is you can't. And, more pertinently, you should never, never try. There is nothing more guaranteed to make the entire internet roll its collective eyes with a loathing just short of actual pity than a too obvious plea to "go viral" - except perhaps some explicit injunction appended to your YouTube clip to "go viral".
Often the reason clips do in fact begin virally infecting a social network or twelve is because, secretly, they have some of the cleverest, most expensive marketing people in the world making sure it does so, behind the scenes. Without telling anyone. But that is not what you want to hear.
At the end of the day, films get viral because people want to share them. And people want to share stuff that is startling, funny, surprising, unusual, compelling, and/or original. Think of things you've found yourself sharing on Facebook or Twitter recently, and what made you want to share it.
Oh, and being topical doesn't hurt of course. If your film happens to chime with some current event, some zeitgeist buzz that is already getting hashtagged on Twitter, pinned on Pinterest, and liked on Facebook, then that'll already give it a bit of a running jump. Just make sure you're tagged with the relevant memes and cross your fingers. Of course, too brazen an attempt to leap on the latest bandwagon and we're back to paragraph one. Despite appearances the internet is not stupid - or at least you should never treat it as though it is.
Finally, you'll want everyone who is already on your side, your friends, your fans, your family, to get behind whatever is you're doing. And you don't want them to do so as some misguided favour to you. That wont carry them very far. You want them to do it because they believe in what you're doing and because they think they'll look cool-by-association through the simple act of sharing your work. That way friends and family become converts, apostles, hosts carrying your virus and spreading it, and making new converts of everyone they spread it to. And so on, and so on ...