I'm sorry, but I'm going to answer your question by telling you that you're asking the wrong question.
Sure, there might be the rare, extremely-occasional movie that is kinda cool, that doesn't involve a cast. But that's weird, and unusual, and actually, I can't think of one. Working with a cast is as essential to filmmaking as air is to breathing.
The question you should be asking is how you can start forming a volunteer cast.
Step #1: ask family
Step #2: ask friends
Step #3: ask acquaintences
Step #4: beg family
Step #5: beg friends
Step #6: beg acquaintences
After you've successfully made your first short film, you'll surely find that the process of gathering a cast has changed:
Step #1: Tell friends and family that you would definitely like to have them again, after they tell you how much fun they had, and that they want to join your next project.
Step #2: Ask even more friends and family.