I finished a 23 minute short in October where the 11 "actors" were exclusively volunteers, mostly friends, friends of friends, and three people I found through an ad in the paper.
My experience is that very few people will say "no, I don't want to be in your film." Almost everyone says yes - and then doesn't show up. My advice is have at least two choices for every role, because your first choice, like this girl you've mentioned, might turn you down.
Also, something I wish I'd done, was have rehearsals. On camera or not, it doesn't matter, I'd just have one rehearsal session for one reason alone: to see who shows up. If they don't show up for rehearsals, they're not going to show for the real thing, so look for someone else.
This girl you want in the film might say yes but then not take it very seriously and not show up, or create other difficulties. You have to dscover how dedicated she is early on. However perfect she might be for the part, if she doesn't show or makes trouble for you, forget her and look for someone else.
That all may be a little negative, but I just don't want to see someone else take months and months to finish a project for no good reason.
Having said all that, I wish you the best of luck with your film.