1) Mice inspired the film, but it wasn't Mickey or Minnie
It's been widely reported that Chris Columbus (screenwriter for Gremlins) came up with the idea for the story while living in a New York loft overrun with rodents. At night he would hear what sounded like a platoon of mice, and the sound of them disappearing into the darker corners of his home filled him with dread. That dread lived on in the film.
2) Each Gremlin cost between $30,000 and $40,000 to make
First picture the number of Gremlins you see in the movie. Now do the math and realize that these little animatronic marvels bit into more cash filled wallets than co-stars during the film. It was also reported that star Phoebe Cates (Kate in the film) was scared to death of these little creations and stayed as far away from them as possible. Maybe she was just scared of breaking one.
3) Gremlins led to the creation of the PG-13 rating
Though a mainstream success, there was a lot of criticism of the film's content and the level of violence which made it into this PG rated film. Following Gremlins and Indiana Jones and the Temple of Doom, Stephen Spielberg (executive producer here) asked the MPAA to update its ratings, which it did two months later. Spielberg explains…
I went to [former MPAA head] Jack Valenti, who's a friend of mine, and I said, 'Jack, why don't we do a rating called PG-13, which would suit films like Gremlins and Indy 2?' Jack said, 'Leave it to me' and later Red Dawn became the first PG-13 film later that year, and the rest is box-office history.
4) That Gremlins connection to Ghostbusters
Warner Brothers had originally scheduled Gremlins to be their Christmas 1984 anchor movie, but when they heard about Columbia Pictures' new Ghostbusters, they felt the need to compete against the film and pushed Gremlins up to a summer release rushing it a bit into theaters.
5) That Gremlins connection to Back To The Future
Kingston Falls is the wonderful mid-American town that is ransacked in Gremlins. That same town also appears in film as Hill Valley in Back to the Future as both films were shot on the exact same set. Great Scott!
6) Originally Gizmo and Stripe were a single character
When the first draft of the screenplay was presented, our pint-sized, cuddly hero Gizmo would later transform into the menacing Stripe, taking the character from lovable to homicidal. Spielberg nixed the idea, feeling that Gizmo was just too cute to turn evil and making the change to have Stripe pop out of him instead.
7) Did you notice the Chuck Jones cameo?
Legendary animator and cartoon artist Chuck Jones made a cameo appearance in the film. He’s the individual, referred to as Mr. Jones, that Billy shows the caricature of the town miser to in Dorry's Tavern. Jones is a cornerstone of the Looney Tunes and Merrie Melodies shorts for Warner Brothers.
8) Some of the planned death scenes were MUCH more grizzly
Good old Mr. Hanson, the science teacher, falls victim to a needle in the tail. Originally, he was to die thanks to a dozens of hypodermic needles to the face but Spielberg though it was just too harsh a scene. Similarly harsh and cut was a scene where Billy's mom climbs up into the attic. There, a gremlin was scripted to cut off her head and toss it back down the stairs to Billy. That death (along with the Gremlins eating the family dog Barney) were similarly removed.
9) The film almost looked a lot different!
Tim Burton was considered and almost had the director's chair for Gremlins. He lost the job because Spielberg worried that at that point he'd never done a full film yet. Can you imagine what it all would be been like if this had been his first film (rather than Pee-Wee's Big Adventure a year later)?