There are lots of people on sites like Kickstarter trying to get documentaries or movies funded (
https://www.kickstarter.com/discover/categories/film%20&%20video?sort=most_funded), you could probably learn a lot by looking at how they gather supporters. Of course every project is different but you can see a lot of the successful ones start by offering unique insight on something mainstream media is less likely to cover or start with an already existing influencer or fanbase. They normally have a lot of graphics and footage (or trailer) before they start trying to raise money.
People willing to fund a movie with Bitcoin are most likely people already involved in the cryptocurrency community, so by appealing to the interests of this community you would in theory be able to start building a support base here, but if you aren't already an accomplished director you'll have to convince people first!
And if you wanted to get big investors involved you'd have to think about how the movie is going to be worth investing in for them (Do they get to advertise before/after/during the movie? Do they get their name in the credits? Do they get money back if the film is successful? etc). And as for actually making the film, you'd have to find a team willing to accept BTC as payment, or convert it to fiat.
By the way, in the cryptocurrency space there was an ICO for a film & tv media distribution company that talked about doing an original television series (SingularDTV:
https://www.smithandcrown.com/sale/singulardtv/) which might be of interest to you.