No matter how many times you've watched the 1985 sci-fi classic, there's always something new to look for in Back to the Future. Writer-director Robert Zemeckis and screenwriter Bob Gale packed the movie with thoughtful details, from Doc Brown's foreshadowing of Back to the Future Part II to a subtle homage to Stanley Kubrick. As Express reports, one of these easy-to-miss details is a short dialogue exchange during the climax, and once you start paying closer attention to it, it could change the way you watch the rest of the movie.
Redditor amanwhodrinks recently laid out his observation in the r/FanTheories subreddit. According to him, Doc's decision to heed Marty's warning from 1955 and take steps to save his own life in the present isn't the abrupt character change it seems to be.
In the past, Doc worries about how time travel might affect the future, and he refuses to hear what the future has in store for him out of fear he might disrupt the space-time continuum. In the present, Marty finds that Doc ended up listening to him after all. When he asks what made him change his mind, Doc responds, "I figured, what the hell."
The Reddit post argues that we see the exact moment when Doc has his change of heart. When Doc is preparing the DeLorean for time travel, Marty explains how his dad won over his mom by standing up to Biff, something he’s never seen his father do before. Doc responds, “Never?” while looking at the restored picture of Marty and his siblings. Marty says, “Yeah, why?” and Doc says “Nothing,” before returning to the task in front of him.
On first viewing, this exchange may not seem important enough to justify a pause in the action so close to the climax of the film, but according amanwhodrinks, it’s there for a good reason: “My theory is that Doc had a major epiphany in this scene. At that moment, when he saw the restored picture of Marty and his brother and sister, he realized that you can completely change the past and still have a positive and congruent outcome on the future without destroying the space-time continuum.” If this hypothesis is accurate, Doc’s decision to influence his own fate at the end of the film is part of a developed character arc, rather than a snap decision made just for the hell of it.
This interpretation is tame compared to some of the theories that have been built around Back to the Future. Some creative fans think that the trilogy is an elaborate chiasmus and others say that Doc Brown is suicidal.