Michael J. Fox was the original choice for the role of Marty McFly…but he wasn't the first actor to play him, thanks to some scheduling conflicts. (Source)

Thomas F. Wilson, who played Biff, did some improvising on set, including coming up with the famous line "make like a tree and get outta here." (Source) 

Why a DeLorean? The idea was to make the car appear as if it might be mistaken for an alien spacecraft back in the 50's, so the overall look—and especially those gull wing doors—created the perfect effect. (Source)

Think Marty had a hard time getting anyone to like his music? How about the BTTF script getting rejected over 40 times before it was finally picked up? (Source)

The time machine was originally not going to be a car at all, but an old refrigerator. Unfortunately, they had some trouble getting it up to 88 mph. (Source)