One early draft of the script had the Mike character as "an inveterate pot smoker who had some sort of memory disease, and looked it. A fact which he used to control his environment and minimize his responsibility." These traits would later be exaggerated and become more tragic for the final film.
When Scott and Mike first appear in Portland, they sit beneath a statue of an elk with an American Indian riding atop it. The statue is real, and still exists in Portland. However, the rider was added by Gus Van Sant. It was really a production assistant covered in grease paint and sitting perfectly still atop the elk statue.
River Phoenix wrote the campfire scene in private, on little scraps of paper. Gus Van Sant was under the impression that he had been writing song lyrics and was surprised when River told him he'd rewritten the scene. Van Sant liked what River presented, but didn't know if Keanu Reeves would be up for it. River assured him that he'd already privately talked to Keanu about it and he was fine with everything. Gus Van Sant says he's he had to give complete faith to River, since he was completely left out of the loop on the whole scene.
At the campfire scene, whenever the camera shows Scott, Mike's hands are hanged to each other, but whenever the camera shows Mike, he is playing with piece of wood.
When Mike is in Idaho he has a clear view of Mt. Hood. Not only is the mountain too far away to see (over 200 miles), there is another mountain range lying between Idaho and Mt. Hood, blocking the view.