Inside the Life and Death of Robin Williams
08/15/2014 AT 05:45 PM EDT
Robin Williams knew how to make a room laugh or cry – and he knew, when opening up last July about returning to TV, what made him happy.
"I thought of a line my daughter once said about me being funny," he said. "When asked, 'Do you laugh?' She said, 'All the time! Even our pets laugh.' "
But recently, laughter was harder to find.
"We knew he was struggling," says a source on the set of his CBS show The Crazy Ones.
"We were doing a scene, and it was just off. I looked over at him, and in that moment, his face changed.
"He looked so exhausted and profoundly, deeply sad. And then one minute later he pulled himself back together, and he nailed the scene. He had a depth; that's where the darkness came from, but there was just so much there."