James Corden had therapy to stop being "a brat".
The 41-year-old star has admitted the fame he found thanks to 'Gavin & Stacey' was "intoxicating" and he didn't handle the attention very well, because he became arrogant and unpleasant.
He said: "I started to behave like a brat that I just don't think I am. It's so intoxicating, that first flush of fame. And I think it's even more intoxicating if you're not bred for it."
The 'Late, Late Show' host's parents prayed for him, and his sisters tried to speak to him about what a "d**khead" he was being.
His mother, Margaret, said: "You can try and say, 'Look, James, you're making a prat of yourself,' but you can only do so much.
"[His sisters] would come in and say to James, 'Don't be a dickhead.' "
And James' more experienced 'Gavin and Stacey' co-star, Rob Brydon, also confronted the actor over lunch.
Rob recalled: "I said, 'Look, this is a bit awkward to say, but I'm just hearing these things about you, and you've got to know that the way you behave has an effect on people.' "
The 'Cats' actor eventually listened and agreed to see a therapist.
He recalled to The New Yorker that he said during his first session: "I used to be a better person than this."
James - who has kids Max, eight, Carey, five, and Charlotte, two, with wife Julia - stopped partying and forced himself to spend more time alone at home.
He said: "The absolute biggest thing I had to learn to do was just stay in and be comfortable on my own."