Bob Dylan's career was in a very weird place in 1973. He hadn't released a new album in three long years, and his last two albums were seen as huge disappointments. Many saw him as a Sixties relic. Not quite sure what to do with himself, he accepted director Sam Peckinpah's offer to take a minor role in his Western Pat Garrett and Billy the Kid. He headed down to Durango, Mexico, to film the movie and cut the soundtrack over a handful of days in January and February of 1973.
The movie was ultimately taken away from Peckinpah in post-production and butchered by the studio, but the song "Knockin' on Heaven's Door" became one of the biggest hits of Dylan's career. It's the story of a sad lawman who feels he can no longer carry out his duties, and it's easy to see how it sort of reflected Dylan's state of mind at the time. It's since been covered by countless other performers, including Guns N' Roses, Eric Clapton and Warren Zevon. It was a regular part of Dylan's setlist for years, though he hasn't touched it once in the past decade.