During the Neary dinner scene, just before Roy piles on the mashed potatoes, the little girl Silvia (Adrienne Campbell) says: "There's a dead fly in my potatoes." This was unscripted and almost caused the rest of the cast to laugh. The scene was kept as-is.
Cary Guffey's (Barry Guiler) performances were so good that they only had to do one or two takes of each shot he was in. He became known as "One-Take Cary" on the set, and director Steven Spielberg had a T-shirt printed up for him with the phrase written on it.
The John Williams score was created before the film was edited. Steven Spielberg edited the film to match the music, a reverse of what is usually done in the film scoring process. Both Spielberg and Williams felt that it ultimately gave the film a lyrical feel.
Douglas Trumbull achieved the dramatic cloud effects by filling a tank half full of salt water with lighter fresh water on top, then injecting paint into the top layer. The paint billowed through the fresh water but flattened out at the top of the heavier salt water, creating the effect we see on screen.
This film is shown every night at the Devil's Tower KOA Campground, thereby making it one of the most screened movies ever.