Exposition is a technique in which background information is conveyed through dialogue, narrative or flashbacks. In the Back to the Future trilogy, the characters, in effect, tell numerous stories as part of the dialogue. By listening carefully, Marty learns information about the past that will help him later when he actually goes into the past in a time machine. Thanks to time travel, much of the exposition of past events can also be viewed as foreshadowing of events in Marty's future.
In Back to the Future, Marty hears three detailed descriptions of facts that he will apply later:
- The clock tower lady tells Marty and Jennifer about how "thirty years ago, lightning struck that clock tower and the clock hasn't rung since!".
- Marty's mother, with the assistance of his sister repeats the story they've heard "a million times", about how her father hit George with the car, she felt sorry for him and decided to go with him to the Enchantment Under the Sea dance, and he kissed her for the very first time. She adds, "It was the night of that terrible thunderstorm, remember George?", and wonders if he had been "birdwatching".
- Doc Brown tells Marty about November 5, 1955: "I remember it vividly. I was standing on the edge of my toilet hanging a clock, the porcelain was wet, I slipped and hit my head on the edge of the sink. And when I came to, I had a revelation, a vision, a picture in my head, a picture of this. This is what makes time travel possible. The flux capacitor." Marty repeats the story back to Doc in 1955 and gains his trust.
Doc also gives information about how he tricked the Libyans, and how Old Man Peabody owned the land on which the mall set and "had this crazy idea about breeding pine trees".
In Back to the Future Part II, Doc and Marty read the USA Today story about Marty Jr.'s arrest, Lorraine tells the story about the crash with a Rolls-Royce, and Biff of 1985A tells about how he got Grays Sports Almanac.
In Back to the Future Part III, Doc's 1885 letter is delivered to Marty on November 13, 1955, telling not only how to repair the time machine, but also giving the details about how Doc settled in as a blacksmith in the Old West.
Doc and Marty read about Buford Tannen in a history book (Buford had been mentioned by the narrator of the film at the Biff Tannen Museum as well).
Finally, Doc lets slip the fact that Marty had gotten himself into "that accident in the future", a reminder of the Rolls-Royce crash referenced in Part II.