Kieran Culkin sleeps for three hours a night

Asking Sarah if she is "keeping up" her health for Variety's Actors on Actors, she told Kieran: "Yeah, I'm not drinking coffee."

He noted: "You did that in London. There was no alcohol."

She replied: "Yeah, and no coffee. If I have a coffee too late, I can't sleep after the show."

Kieran then admitted: "I was going to try to do what you do. Instead, my diet is like five cups of coffee, two scotches after a show and a 2 am cheeseburger."

His friend was shocked by his admission.

She said: "Oh my God. Do you sleep?"

Kieran admitted: "Three hours. I also have sleep apnoea, which is fun to talk about on camera. Sleep apnoea, guys."

Sarah sympathised: "Oh, dude. That's bad for your brain."

The pair played siblings Shiv and Roman Roy in 'Succession' and Sarah, 37, recalled one scene in which Kieran "didn't know" any of his lines, which left her struggling with her own dialogue.

Discussing how they learn scripts, Kieran explained he doesn't know and "just reads" them, while Sarah prefers to "walk".

The 'A Real Pain' star noted: "Sometimes people learn them within the blocking, so if the blocking changes, they can't remember."

Sarah laughed: "That's what I used to do with 'Succession'. I learned 80% of it. Then I would get 100% by the time the blocking was set.

"Except that time that you... I would rely on other people to know their lines as well, and there was the time you came and you didn't know any of them."

Kieran admitted: "I just didn't know any of them and I thought I could do it. I just remember starting the scene and going, 'I actually don't know this - even a little.' "