Sean Penn feared he'd set his children up for "humiliation" when he cast them in 'Flag Day'

He told Us Weekly magazine: "I don't know if this is true of everyone, [but] the greatest joy [in life] is often relief. It's when you take a chance with something, you invest in it.

"And sometimes with so much excitement, you forget the risk that you put yourself or someone else at.

"And when I've asked my son Hopper or daughter Dylan to do something like act in a movie, it's only later you realise, 'Jesus, this could have been a setup for their humiliation.' And then to experience that relief when they're great ... It's exciting to do."

Sean admitted he had some selfish reasons for bringing Dylan and Hopper onto the project.

He said: "I sleep best [when] my kids stay too late at a barbecue that goes into the night and pass out here and I know everybody's in proximity. I feel the same way with work when they're around."

Hopper previously admitted he and his dad used to regularly "butt heads" because of his "strict" parenting but they get on better these days because the 'Milk' actor has mellowed in his "older age".

He told People magazine: "He was strict, and I was always getting into trouble. We butted heads for a long time. But it's very lax now. He's chilled out in his older age!"

Although father and son are close now, Hopper confessed to "rarely" reading his dad's text messages because they are so long.

He said: "My dad is really into FaceTime and texting. But when he texts, they're like novels. He has to send them in increments. So I rarely read them!"

Hopper has more appreciation for his father's work now that he's older.

He said: "'Milk' [is one of my favourites]. And then 'The Falcon and the Snowman'.

"But when he went to Haiti [after the catastrophic 2010 earthquake], I'd never seen that side of him. It was just immediate, and he was really smart with it. Emergency mode is his place. I feel like he's the calmest when chaos is going on. It's really great."