[5] Mark Ruffalo and Laura Dern play one couple, Naomi Watts and Peter Krause play another. Both couples are friendly, especially Ruffalo and Watts, who sneak away every possible moment to have sex with each other. Dern’s no dummy, though. She knows her husband is cheating with her friend — and it’s driving her nuts. Krause knows, too — but he’s just grateful someone else …