I'm in that spot right now casting my friends. It helps that everyone knows me as the film making/storyteller friend, so when I come up with some 'fantastic' project I'm all excited about, no one's surprised XD. What I do is take a serious look at my characters and then a serious assessment of what my friends look like and are capable of. I am in the fantasy/animation genre primarily, so a live action film with my friends will have just as much emphasis on expressions and subtlety as if I were drawing frames. I actually purposefully created the script I'm working on currently to work with my friends' personas.
For example, I have two friends who are black belts in Tai Kwon Doh. One of them is also a breakdancer, who likes the nonchalant ultra-cool guy persona. The other one, when being funny, is very emotive and witty, and even poetic in serious times. Even though they are just normal people, I can take those traits they enjoy showing and exemplify them in the characters. My breakdancing friend is now a badass villain with few (if any) spoken lines. He delivers through his eyes and posture. Meanwhile my emotive friend gets the largest speaking part as the cryptic figure who aids the protagonist. These two have a fight scene against each other, and I can't tell you how excited they are about that
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However, you do have to work with them. Alot. Explaining the character and their role inside and out. I actually act back to them, giving them an example of how they must feel. (I even have to coach them on expressions.) You won't be the one on camera, but if you can show them the beats for their movements physically, then you'll see them start to be proud of the character, and need your help less. Potentially even take off with it better than you imagined. Some people hook on faster than others, especially when the role is tailored for them to like it.
The real dilemma is when you have a story that no one in your friend group fits, no one returns your enthusiasm, or they're just plain unreliable. In which case you may have to go out of your friend group by posting an ad in the newspaper or on a website. "No pay, but there will be fun, food, and potential acting exposure."
Potentially seek out people from a drama club. They will at least have more experience than friends.