In the movie, John Candy plays the role of Freddie, a somewhat pompous and demanding restaurant owner. During a scene where Tom Hanks' character, Allen, enjoys a steak dinner with a mermaid (played by Daryl Hannah), Freddie approaches and, instead of asking how the meal is, he inquires:
"You want dessert?"
Allen, rather surprised and confused, says "No, I'm good, thanks."
The scene is humorous because Freddie doesn't seem to be interested in Allen's actual enjoyment of the meal, but rather in pushing the sale of dessert.
While it's not a scene where John Candy is told to eat anything, it's a classic and memorable moment from the film.