Will people work on short films without getting paid?
Yes. People will work for free if they believe in you and the project. It's just as important for them to get the experience and have material for their demo reel. And it's fun.
If you have the means to pay them, then pay them. If you aren't getting paid and it's just a passion project, then be honest. If they turn you down, say thanks and move on.
Remember this: Respect their time. Take care of them.
That's generally good advice even when you're paying someone, by the way.