He appears in the haunted house at the Fun Fair. Some can be seen outside the windows in Ghost Houses watching the player, but these Big Boos do not do anything special. They can also be put in Clown Cars, Lakitu's Cloud and on tracks. Gadd thought that King Boo was there along with Mario's painting. To defeat Big Boo, Luigi has two different methods of attacking it.
Dieting sucks. It tends to lead to cravings… and hunger. This generally causes people to give up on their diet and gain the weight back.