Cumming and Henderson write 40-year friendship into History of Paper

Their shared history means they're comfortable and convincing as a couple talking – and singing – about their lives together.

"It's just easy to be with Alan," Henderson said.

"We have a good chemistry. We don't over analyse it."

"I come to see you before the show, each night," said Cumming, "and I say, 'let's have fun'. If we can, because it's terrifying.

"Because you've got this incredible roller coaster of emotions. Even in the happy bits, I want to cry."

"You just can't help but feel something," said Henderson.

"It's like when the first snows come. It's a tenderness, but there's a rawness to it as well."

Reporting from BBC News contributed to this story.