Cast the Expert: You know how the dialogue between the pilots and the air traffic controllers sounded so realistic? 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.
Enforced Method Acting: 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.
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.
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.