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.
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.
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.