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.
Since the book was released, hundreds (if not thousands) of fans have made their way to the site where McCandless died and not all of them have survived. On 14th August 2010, a Swiss national 29-year-old Claire Jane Ackermann attempted to cross the Teklanika River with a 27-year-old man, they lost their footing and were pulled under by the current. The man survived. Around the same time two years later, another young fan from Oklahoma, Jonathan Croom, went missing for some time before his dead body was found in the mountains of Oregon. His father later stated that Jonathan had very limited camping experience before he embarked on this treacherous journey. Another McCandless fan from Oklahoma, 19 year old Dustin Self, also went missing in March 2013 and his dead body was found on 7th October 2014. His last communication was with his father and ex-girlfriend when he called to tell them he was lost in a remote area of the southern part of the county.
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."
In the film, Chris dies from mixing up two very similar types of plants: one edible and one poisonous. In the final edition of the book, Into the Wild, John Krakauer explains that although this was the original belief behind Chris' death, he has found evidence that strongly supports that Chris knew the difference between the two plants, and instead starved due to eating a deadly mold that had grown on the plant.