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 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.
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.
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.
The scene in which Jillian sees the image of Devil's Tower on TV while staying in a motel was not in the original shooting script. As written, Jillian was to have remained in her house, shut off from the outside world. During production, however, Steven Spielberg decided it was better to show that Jillian had traveled out west searching for her missing son, and so the set of a New Mexico-style motel room was hastily built to film the scene.
J. Allen Hynek: Famed ufologist, who coined the phrase "close encounter of the third kind". He can be seen as the gray-haired man with glasses, a pointed beard, and a pipe walking out to see the returnees in the final sequence.