Jude Law "cried at anything" when he became a parent.
The 47-year-old actor - who has six children - admitted he always draws on his relationship with his kids when he has to work on something "emotional".
He said: "[When I became a parent] I started crying at anything. Everything emotional immediately goes to my relationship with them - and the capacity of love and the capacity of loss and all of that."
And playing troubled dad Sam, who is dealing with the death of his child, in new drama series 'The Third Day', prompted him to repeatedly tell his kids how much he loves them.
He told The Guardian newspaper: "It's a sort of muddy area. One doesn't sit dwelling on the loss of your own children, but you kind of poke around at what that might be.
"My children all got lots of messages during the making of this, telling them how much I loved them."
Jude admitted he "underestimated" how draining it would be to take on the role, particularly as Sam seems to lose his grip on reality.
He said: "I completely underestimated getting into Sam's skin.
"I think messing with grief, really sticking your finger in and poking around at what grief is ...The emotion is unrelenting for Sam."
'The Third Day' features three episodes, with the second being broadcast live for 12 hours and while he's accustomed to live performance thanks to his theatre work, Jude still thinks it will pose a challenge for all involved.
He admitted: "I've never done anything, or stayed in a character, for such a long period of time."