You should never under-estimate the power of flatulence. It made for an iconic scene in Good Will Hunting, and it also had a huge part to play in the development of one of The Usual Suspect's most well-remembered.
The line-up scene was intended to be serious, but a full day of filming had left the actors distracted and hysterical, so Bryan Singer instead decided he'd use the funniest takes. And the reason for their hysteria was revealed later when Kevin Pollack confessed that Benicio Del Toro had "farted, like 12 takes in a row," cracking everyone up.
Elsewhere, Singer encouraged his actors' performances by telling each of them that they were Keyser Soze, while Peter Greene got legitimate reactions out of both Stephen Baldwin and Benicio Del Toro when he threw a cigarette into the former's face by accident. It was meant to hit him in the chest, which is why Baldwin and Del Toro's shock and panic are so real.
Which other real moments of non-acting belong on this list? Share your own favourites below in the comments thread.