I think you may have suspected this toward the end, but the reason it was so hard to "call" what's going to happen next, and the reason all the characters were so complex, is that the movie is based on real people and real events. Doc Holiday? Real guy. Also Virgil, Wyatt, and Morgan Earp. The Gunfight at the OK Corral was a big thing, and this is far from the first western movie to portray it.
So, if you're interested in a prequel--well, there isn't a movie, but there's Wikipedia. :)