Eric Stoltz was the original Marty. After filming quite a few scenes they realised he was being a bit too dramatic, so they cast Michael J Fox.
In the very beginning of the movie, there's a cute little gimmick. In the first scene when Marty arrives at Doc's place and all the clocks are being shown, they show one particular clock (amongst hundreds), but this particular clock has an hour hand and a minute hand and there is a man hanging off one of the hands - foreshadowing Doc's perilous scene hanging from the clock tower much later in the movie. The man isn't actually Doc, but the silent movie star Harold Lloyd, but still a deliberate inclusion, and worth a look.
Melora Hardin was originally cast as Jennifer Parker (Marty McFly's girlfriend). She was later let go when Michael J. Fox replaced Eric Stoltz (who was originally cast as Marty McFly) because she was much taller than Fox and director Robert Zemeckis thought that they would look odd together on screen.
In 1985 Doc is living in what was in 1955 his garage/workshop after the Brown mansion burned down (note the newspaper clippings at the start). Looking at the appearance of it in the different years you can see that they are same place. This also explains why Marty looked at the garage so intently when he first arrives at Doc's house in 1955, he must have been surprised to learn this.
Originally, the beginning of the film started out with Marty sitting in class at school. He is caught listening to his walkman and sent to Strickland's office, causing him to be late for the battle of the bands audition. This was filmed with Eric Stoltz, who was cast first as Marty. When Stoltz was fired and Michael J. Fox was hired, the beginning was quickly rewritten to start in Doc Brown's place, with Marty being late for school.
When Marty returned to 1985, Red was on a park bench that had the logo for California Raisins. The California Raisin Board paid the producers $50,000 for this product placement. However, since it was so dark and dirty, and since Red was covering up most of the logo, half their money was refunded.
At 1:15 am on October 26, 1985, a crowd of people showed up at the mall that was used as the Twin Pines/Lone Pine Mall for the movie, which had come out months before. They wanted to see if anything peculiar was going to happen. Nothing did.
Director Robert Zemeckis and producer Bob Gale's first choice for the part of Marty was Michael J. Fox. When he was unavailable due to scheduling conflicts, their second choice was C. Thomas Howell but the president of MCA wanted Eric Stoltz for the part. Six weeks into production, Zemeckis fired Stoltz and was then able to get the then available Michael J. Fox for the part.
The amplifier that Michael J Fox blows up at the beginning is labeled as being a CRM-114. This is a reference to Dr. Strangelove, in which the B-52 crews receive their orders over a CRM-114. It was also the serial number of the Jupiter explorer in 2001: A Space Odyssey (1968) also directed by Stanley Kubrick.
This movie is Billy Zane's first screen appearance. He plays one of Biff's side kicks. He's easiest to see just after Biff calls Marty's dad an 'Irish bug'. Marty stares, then Biff grabs him to say 'you finished my homework yet?' He can also be seen just after Marty runs over the convertible later, kneeling in the back of the car before it crashes into the dung van.