California Doubling: Mobile, Alabama, stands in for Muncie, Indiana. Additionally, nearby Bay Minette, Alabama, stands in for Moorcroft, Wyoming. Some scenes were filmed at the real Devil's Tower, though the climax was, of course, filmed on a sound stage... an aircraft hangar in Mobile, Alabama. The Ohio state line was filmed on the approach to the Vincent Thomas Bridge, in Long Beach California.
Cast the Expert: You know how the dialogue between the pilots and the air traffic controllers sounded so realistic? Steven Spielberg used the controllers from Los Angeles ARTCC, gave them the scenario, and they played it as if it were real.
Also, the synthesizer player is Philip Dodds, one of the lead engineers for ARP Instruments. Dodds was brought in to install the ARP 2500, and when Spielberg saw his expert handling of the equipment, he offered Dodds the role on the spot.
The "voice" of the mothership was performed by legendary tubist Jim Self. Spielberg cast him and his performance because the difficulty of playing the tuba added a "human" component to the alien communication.
Christmas Rushed: Originally intended as a Summer 1978 release, a debt-ridden Columbia Pictures ordered Spielberg to complete it for the 1977 Holiday season so the studio could stave off bankruptcy. The rushed post-production prompted Spielberg to Re-Cut the film later on.
Deleted Scene: A deleted scene showed Roy watching the sky from an observation platform he had constructed on the roof of his house. The platform itself can still be spotted when Ronnie backs out of the driveway in the family station wagon.
Duelling Movies: With Star Wars, the other big science-fiction blockbuster that year. Both proved to be game-changers for the genre.
Used with the young actor playing Barry. For example to get good reactions in the kitchen scene, where you only see his face, off camera they had a clown which Barry (Cary Guffy) smiled at. Then (same take)they brought in a man in a gorilla suit, which he looked at with a puzzled expression. Then the man in the gorilla suit took his mask off, revealing it to be a grown up he knew. Cary smiled.
Also, he said "Toys" because he was actually shown a bunch of toys to get him to smile.
According to Melinda Dillon, because it was done without rehearsal, the scene in the kitchen with all the objects flying around was truly scary, and her alarmed reactions were often quite real and spontaneous as she tried to protect herself and Cary Guffey.
Melinda's Oh, my God! is real, as she (and we) first see the Mothership. Spielberg told her he would speak to her as if he were D. W. Griffith and she were Lillian Gish. He kept talking, telling her she was about to see the most beautiful, wondrous thing ever — and when he finally said "turn around", all he had was a piece of paper — but she'd imagined one of her first memories, the face of her grandmother leaning over her crib.
Executive Meddling: Lots of footage was edited around and cut in the original 1977 version. Justified as the studio was on the verge of bankruptcy and needed to ship this film before they went under.
Mental Illness and what it can do to nice suburban people. Lots of men do in fact leave their families. One parent losing his or her mind is a common cause of divorce. In this case Neary's craziness is literally the result of alien mind control and his family wisely flees from him to avoid further exposure to his psychotic behavior.
And specifically, what has happened to thousands of people who believe, in all sincerity, that they truly are alien contactees. MUFON meetings are full of people who left/were left by their families because of an Experience.
Never Work with Children or Animals: The aliens were played by young girls, who were hyperactive and incredibly difficult to control. In Bob Balaban's production diary, he said that Steven Spielberg often began takes by yelling "Girls, stop disco-dancing!".
Real-Life Relative: Richard Dreyfuss's father was an extra in the film and spent six months on location; however, the scenes in which he appeared never made the final cut. Meanwhile, Roy's son Toby was played by his nephew Justin.
Throw It In!: During the dinner scene just before Roy piles on the mashed potatoes, you can hear the little girl say, "There's a fly in my potatoes." This was unscripted and almost caused the rest of the cast to laugh. The scene was kept as-is.
Technology Marches On. To cite just one example, the scene with the giant globe would make no sense in the era of Google Maps. Also, Spielberg himself says that UFO phenomena was more believable in the '70s, before camcorders were invented in the '80s and sightings actually went down.
A middle-aged man mentioning that he saw Bigfoot in 1951 and 30s-ish Roy giving 1944 as his birth year.
Major Benchley says, "Ladies and gentlemen, this is a flying saucer. It's made of pewter, made in Japan, and thrown across the lawn by one of my children." This carries the implication of Japan being a source of cheap goods, which it was back then.
One early concept for interpreting the aliens included an orangutan on roller skates. The idea did not work, because the orangutan became very frightened the second its roller skates touched the ground, and it kept grabbing onto the arms of its caretaker.
The film originally ended with the version of "When You Wish Upon a Star", but it tested negatively in previews and was cut (this detail remains on the novelization, though, with Roy remembering the lyrics in an inner monologue). That is why Roy Neary was trying to convince his family to see that film together just before the blackout. The song remains incorporated in the score, though. A toy can also be heard playing the song's melody right before Roy rips off the top off his sculpture.
Spielberg approached Steve McQueen about playing Roy Neary. [McQueen] declined, as he couldn't cry on cue. Gene Hackman turned it down because he was going through a divorce at the time and Jack Nicholson turned it down because he feared that the effects would upstage the actors. Dustin Hoffman and Al Pacino also passed on it.