In the scene where Sean starts talking about his dead wife and her farting antics the lines were ad-libbed by Robin Williams, which is why Matt Damon is laughing so hard. The scene took everyone by surprise. According to Damon in the DVD commentary, this caused the cameraman to laugh so hard that the camera can be seen moving up and down slightly.
Today I put my hand in my backpack and felt a stress ball and I was like “oh? I have a stress ball?” and I squeezed it and it was a pear and it exploded and now I’m much more stressed than I was earlier :/