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In Back to the Future Part III (1990), several notable scenes and lines were improvised or ad-libbed by the actors. Director Robert Zemeckis and writer Bob Gale often encouraged the cast—particularly veterans like Christopher Lloyd and Thomas F. Wilson—to add spontaneous touches to their performances.
Below are the specific improvised moments with detail:
One of the most famous improvised moments occurs when Marty (Michael J. Fox) wakes up in the McFly farmhouse in 1885. After being knocked unconscious and rescued by his ancestors, Seamus and Maggie McFly, Marty wakes up in bed. In a deliberate but unscripted callback to the original 1985 film (where he wakes up in his mother’s 1955 bedroom), Michael J. Fox improvised the gesture of checking under the blanket to see if he was wearing pants. This was not in the script but was added by Fox on the day of filming to mirror the iconic scene with Lorraine from the first movie.
The three "old-timers" sitting in the Palace Saloon—Levi, Zeke, and Jeb—were played by legendary Western character actors Dub Taylor, Pat Buttram, and Harry Carey Jr. Because of their decades of experience in the genre, Zemeckis allowed them to ad-lib much of their background commentary to give the scene authenticity.
Thomas F. Wilson, who played Buford "Mad Dog" Tannen, was a frequent improviser throughout the trilogy (he famously invented the "Butthead" insult and "Make like a tree and get out of here" in Part I).
The rock band ZZ Top made a cameo as the town band at the Hill Valley Festival. During their performance of "Doubleback," they performed a synchronized 360-degree flip of their instruments (guitars and even the drum). This was not a choreographed instruction from the director; rather, it was a signature move the band did spontaneously on set. Zemeckis liked it so much he kept it in the film, and the camera captures a genuine look of surprise on Marty’s face as he watches them.
During the scene where Doc Brown (Christopher Lloyd) is knocked out by a single shot of whiskey and the bartender prepares "Wake-Up Juice," Lloyd and the bartender (Matt Clark) improvised several of the physical beats. Lloyd’s "stiff-as-a-board" fall and the various facial contortions he made as he was being revived were largely developed through physical improvisation during rehearsals and filming to heighten the comedy of Doc’s low alcohol tolerance.
Mary Steenburgen (Clara Clayton) noted in interviews that she and Christopher Lloyd were so enthusiastic during the filming of the festival dance that they began improvising their movements and energy. This spontaneity actually led to a minor injury; they were dancing so hard that Steenburgen accidentally tore a ligament in her foot, though she finished the scene and the "overly zealous" energy remained in the final cut.
While not an 'improvised scene' (it was a stunt failure), the hanging scene is the most significant unscripted event in the film where Fox actually passed out, often discussed in the context of 'real' moments in the film.
In Back to the Future Part III, confirmed improvisations include Michael J. Fox ad-libbing the physical comedy of falling out of bed (though the 'pants check' was a scripted callback) and Mary Steenburgen and Christopher Lloyd dancing with such 'overzealous' energy that Steenburgen tore a ligament. Thomas F. Wilson, known for improvising Biff's catchphrases in earlier films, likely ad-libbed aspects of Mad Dog Tannen's aggression. However, the 'Dead Chinese' line was scripted, and ZZ Top's instrument spin was their signature move, likely requested by the director rather than a spontaneous surprise.