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.
This clip is probably my earliest memory of Robin Williams - it’s always stuck in my mind for some reason. As I was searching for this video I was trying to remember what topic he was talking about (all I could remember were the hilarious banana/shoe comments that my very young self thought were the funniest things) and then the video started and, well…I didn’t see the title and was caught completely off-guard. Rest in peace you magnificent man.