One of my most rewarding experiences as a director came from casting a lead that I wasn't sure could handle the part - a true greenhorn that I literally had to teach stage direction before we could even begin.  In the end he did a great job.  He wasn't the best I've ever seen.  I had to work with him a lot.  He completely messed up some of my blocking in a few places.  But all in all it was a great show, everyone else was really strong and several people say it was the best show they've seen by this theater AND veteran cast members say it was the most fun they've ever had in a show.