Marvel Cinematic Universe fans know Mark Ruffalo best as Bruce Banner, aka The Incredible Hulk. But he's way more than just the guy behind the big green guy. Ruffalo is one of the most sought-after actors in Hollywood, appearing in award-winning prestige TV and arthouse films, when he's not supporting various charities and causes, like clean energy. But there was one key moment back in 2001 — right after his breakout turn in the Academy Award-nominated "You Can Count on Me" opposite Laura Linney — that Ruffalo cites as truly life-changing. This is the one tragedy that changed Mark Ruffalo forever.