Audiences might have a problem with how the movie dramatically shifts gears every few minutes—scenes of light stoner comedy are immediately followed by scenes of intense violence—but it's not a mistake on Nelson's part. Leaves of Grass has the structure and the elements of a classical Greek drama: There's treachery, mistaken identity, deadly plots, and ambition; that it takes place in the middle of Oklahoma is almost irrelevant.
Norton convincingly sells both parts as his own twin: His Okie pot dealer isn't played just for red state shits and giggles (we're reminded a couple times that he's smarter than his brother) and his philosophy professor isn't just a hoity-toity straight man, either. He holds the film together while confidently stealing the spotlight from himself on several different occasions. Leaves of Grass is all done at SIFF, and Norton admitted after the screening* that the film hasn't picked up a distributor. Hopefully, you'll be able to see it before this time next year.
* I don't have much to say about the audience Q&A with Norton—the first question, literally, was "Why are you so awesome?"—but he did seem like an earnest, awkward guy. (Norton's answer to the "awesome" question, by the way, was, "I'm going to assume that that's a rhetorical question.") He made a point out of saying that most film festivals are awful commercial messes, and that SIFF was a wonderful festival for a "great film town," which I thought was a nice thing for him to say.