Sean Penn needed wolves during production, and even rented some from an agency. Penn played "kissy face" with several of the wolves before, during, and after production. He spent hours feeding, playing with, and petting them. Several of the wolves are still alive, and on display at Wolf Country USA in Palmer, AK.
The trivia items below may give away important plot points.
The note that Christopher Johnson McCandless tacked to the bus actually said, "S.O.S. I need your help. I am injured, near death, and too weak to hike out of here. I am all alone, this is no joke. In the name of God, please remain to save me. I am out collecting berries close by and shall return this evening. Thank you, Chris McCandless. August?" The note shown in the film was a page from one of his "journals."
Shot on location, except for the bus scenes. According to Sean Penn they abandoned the idea of shooting at the real bus out of respect for Christopher and the McCandless family. Instead, they built a set in the wilderness, with an exact replica of the real bus.
Though the book on McCandless's life and the movie it spawned were sympathetic to the whole situation, many Alaskans believe that he was foolish to embark on such a lifestyle without the appropriate skills or equipment, such as a map or compass. Alaskan Park Ranger Peter Christian has said, "When you consider McCandless from my perspective, you quickly see that what he did wasn't even particularly daring, just stupid, tragic, and inconsiderate. First off, he spent very little time learning how to actually live in the wild. He arrived at the Stampede Trail without even a map of the area. If he had a good map he could have walked out of his predicament. Essentially, Chris McCandless committed suicide."