In any case, I thought Rio Bravo was a truly delightful film. Although not particularly significant in the evolution of the genre, it has an incredible sense of fun to it because the characters, all of whom have nicknames such as "feathers," "stumpy," "Colorado," and "Dude," are so colorful. One could observe that Walter Brennan's "Stumpy" is the comic relief in the group, but one stroke of genius is that every character has a little bit of humor in them. Dean Martin (playing the alcoholic sidekick "Dude"), at times, plays the straight man to Stumpy or John Wayne, like he often did on the Dean Martin show. Martin is an unlikely choice but I think it's pitch perfect casting. Angie Dickinson and John Wayne made a good comic pairing as well. Watching Stagecoach and Searchers it's hard to fathom Wayne having a lighter side, but he does.
If I had to think of a theme to Rio Bravo it might be that of having to act in the present. Wayne's doubled by Colorado as the naive kid who he used to be, Dude as a reminder of how he could have turned out if he had so easily fallen into temptation, and Stumpy as the old crippled man he will turn into one day and the responsibility falls on Wayne to act decisively in the moment as opposed to those whose moments have not yet arrived or whose moments have passed.
I also see a reverse of the classic discourses in the Western as shown in films like Stagecoach and Shane, where civilization is neatly divided between the civilized, the wilderness, and the Western hero who has to mediate between them. In Stagecoach, the civilized characters (i.e. the banker, the Southern gentleman, the cavalry man's wife) turn out to be useless, while the town drunk, the prostitute and the fugitive are the ones who save the day. In Rio Bravo, the day is metaphorically saved. Civilization has been built and the rough-and-tumble fugitive (played by the very same actor) is now sheriff of the town, and he slowly arrives at herosim by sifting through the people around him and recognizing the value of the town drunk (Dude) and prostitute (Feathers). The allusion to the fact that Feathers was a prostitute at the end is the exact opposite of Stagecoach where we learn that Dallas is a prostitute first and a maternal woman with motherly and domesticated instincts second.