Witness Easter Egg - Great Cup of Coffee

In the movie, Harrison Ford is shot and is treated by an Amish family (Kelly McGillis, Jan Rubes, Lukas Haas). When he recovers, there is a scene where the family and Harrison Ford are having breakfast. After a while Harrison Ford takes his cup of coffee, points with his finger at it and says: "Honey, this is a *great* cup of coffee".
Kelly McGillis and Jan Rubes look at Harrison Ford, but continue their breakfast and Harrison Ford mumbles: "Some TV-commercial for coffee".

This scene wasn't in the script, but Harrison Ford wanted to play a practical joke on the other actors.

At the start of his career, Harrison Ford had done some commercials, one of them for a coffee brand. In that commercial, he had to take his cup of coffee, point at it with his finger and say the above line.

By playing that scene, Harrison Ford thought that Kelly McGillis and/or Jan Rubes would break into laughter and the scene would have to be replayed. They didn't, but instead continued their roles and looked at Harrison Ford like Amish would do. So Harrison Ford was forced to mumble some explanation, which he did.