Location! Yes, location is so very important in any film, but with no budget, low budget films, it's takes on a whole new level of importance. Why? Because you can only afford to have one or two major locations. You won't be able to afford more.
Want to know more? Check out my "How To Make a Feature Film For Under 25K."
I shot SLEEPWALK on 35mm and we had one major location, and got it in the can for under 50K! And that was 35mm! It's much cheaper now with digital DSLR's.