Plenty of accidents happen on set, but most of them are simple trips or flubs. In “Back to the Future Part III”, however, something much worse almost happened. At one point Marty is dragged behind a horse and hanged.
Because Michael J. Fox wasn’t standing on a box – his own choice in order to make it more realistic – after a few takes he got tired and couldn’t get his hand under the noose in the right spot to keep his airway open. He actually passed out for a few seconds before the filmmakers noticed. He later recounted the incident in his autobiography “Lucky Man”.
The McFly Boys Have a Type
One of the odd problems people have with "Back to the Future III" is that Lea Thompson, who plays Lorraine Baines in 1955, also plays Maggie McFly, the woman married to Marty's ancestor Seamus McFly.
While it makes sense for Michael J. Fox to play both Marty and Seamus, it's a little strange for the same actress to play people not related to the McFlys. The reasoning the filmmakers went with was that it wasn't too much of a stretch for the McFly men to all be attracted to women with similar features – something that is somewhat true to life.
Just Let Him Drink
The series has plenty of recurring gags, and one of the first that we see in each movie is Marty unable to finish his drink.
In all three films, he wanders into the local watering hole and gets a drink, be it a fifty-dollar Pepsi or a shot of whiskey. But every time he's about to bring the drink to his lips, the antagonist of the film – be it Biff, Biff's son, or Mad Dog Tannen – barges in and calls out his name. Marty is never able to wet his whistle, though in the case of the whiskey it's a good thing.
Switching Things Up
After two full films, we've gotten used to the catchphrases of the two main characters. Marty McFly calls things heavy – a very eighties phrase – and Doc Brown likes to exclaim “Great Scott!” By the time we get to the third film, the characters and the filmmakers decided to swap things around.
The characters have at this point become so close that, after one shocking scene, Marty yells out “Great Scott!” immediately followed by Doc Brown saying: “I know, this is heavy.” After two whole films of establishing the characters, it's fun to see them swap phrases for a one-off joke.
“I Hate Manure!”
An ongoing gag from all three movies is the unfortunate way that Biff (and Mad Dog) keeps ending up in manure. All three movies feature a scene of this kind, and the version from the third film even features an additional easter egg.
In the first two films, it's due to vehicular misadventures that land the bad guy in the bad stuff, but in "Part III" it's because of Marty's heroic gunfight and fistfight with Mad Dog. When Marty lands the knockout hit, Mad Dog spins and passes out just like Biff did when George McFly punched him near the end of "Back to the Future".