The actor plays author Blake Morrison watching his dad's last moments and reflecting on their relationship.

Firth terrified by fatherhood

Firth says after his own child was born his fatherhood fears faded: "I didn't find it frightening after I became one. I think I just found it distasteful. It didn't appeal to me, that's all."

The film is a sentimental look at a boy and his father's difficult relationship interspersed with moments of joy and bliss.

Firth says the film taps into a primal theme: "There's a great line by the writer Robert Towne about fatherhood which hit the nail on the head for me.

"He said he'd always associated fatherhood with age and the atrophy that goes with comfort — pipes and slippers and eventually death."

Father of three Firth, 47, insists that having his own children was the best thing he ever did.

"Having a baby was rejuvenating and wild and wonderful. Being a father is more like passionate love than I'd imagined. You have the same sense of being on the brink of being out of control, and of utter euphoria.

"It's what makes life most worth living — no question. Fatherhood is a more important challenge than acting."

And When Did You Last See Your Father? looks at how important children are to even the most wayward father.

Firth says in his own life his children are more important to him than his career: "It runs far deeper into what I care about most. If I had to do without acting, I'd survive.

"But I simply couldn't do without my kids. I change the nappies and help out wherever I can, but I think most dads do that. Even Tony Blair does."

Firth has become a sex symbol after roles in hit films including Bridget Jones's Diary but admits in real life he's not like that.

He's married to Livia Giuggioli and says: "I don't think she thinks I'm the strong, silent type in real life.

"I'm more of a nerd. I'm a fairly dorky sort of person. If I went around trying to smoulder at people in real life, they'd just laugh at me. Not all Englishmen are repressed though."