He was friends with the lead actor, he met Moco while raising the money. And he has explain that he would cast people but never let them read the script and only shot them on a couple of days over the course of the shoot. Just gave them the pages for the day and only borrowed them for a few hours so they would feel like they weren't doing a beep ton.
The other people he would just simply ask. Such as with the guards at the jail, the people who owned/worked the bar were the ones drinking at the bar and getting shot. He shot short films in that town previously so he had at least a small relationship with the town and some of the people.
Utilized what and who you know in short.

Rodriguez doesn't care about actors. All he cares about is doing things cheap, visual effects, cheap marketing gimmicks, stunt casting, making unnecessary sequels etc etc etc. Because who needs actors when you can have those?
In all seriousness, I don't think I've ever seen him talk about actors at all in any sort of interview or DVD commentary.