Richard E Grant has his mother to thank for his assiduous diary-keeping - one of whose by-products has been the enjoyable With Nails journals about the making of everybody's favourite Richard E Grant film, Withnail and I. Not that his diary-keeping received any maternal encouragement. For, as Grant's new movie - his semi-autobiographical directorial debut, Wah-Wah - makes plain, his mother was not that sort of a woman. Instead, it was the traumatic experience of witnessing her infidelity that first shocked an 11-year-old Grant into recording his thoughts and feelings on paper. While pretending to be asleep on the back seat of his mother's Mercedes, he saw her make love to his father's best friend.
"I started keeping a diary that very day and have done ever since," he tells me now. "I tried God and never got a reply so I gave up on that after a couple of years. The diaries kept me sane."
Wah-Wah is set in late 1960s Swaziland, in the dying day of British colonial rule, where Grant's father (played by Gabriel Byrne) was minister of education. In this "last outpost of the Union Jack on the African continent", the British went around speaking in anachronistic "toodle-pip, old boy" voices that were satirised by Grant's future stepmother, an American air hostess played in the film by Emily Watson, as "wah-wah".
In this "bizarre, claustrophobic, eccentric expat universe", Grant's cuckolded father descended into self-pitying and sometimes frightening alcoholism. There's a scene in Wah-Wah where his father (called Harry Compton in the film) points a loaded revolver at the young Grant (played by Nicholas Hoult from About a Boy) after his son tips away a crate of whisky. He fires and only narrowly misses. "I thought I was going to die. The bullet whistled past my head," recalls Grant, who underwent 18 months of psychoanalysis when he was 42, partly to come to terms with the crippling abuse his father threw at him while drunk.
But while attempted filicide seems barely credible, there were other events that Grant excised from the film because he felt audiences would find them too bizarre. "At my father's funeral the young Swazi priest, who had just come back from an evangelical course in America, jumped into the grave, undid the lid of the coffin and said: 'I am going to raise your father from the dead.' It was Monty Python meets Joe Orton.
"I have to say the film is semi-autobiographical because it's 10 years that have been concertinaed down into three years for the story. But everything you see happen did happen, and my family are all pretty verbatim."
Grant's first problem was assembling a cast. "Because my father was 42 when this happened, I went to all the usual suspects of that age. There are a handful of English men in their forties who can get a film financed and I went to every single one of them - Daniel Day-Lewis, Colin Firth, Hugh Grant and Rupert Everett, who couldn't see himself as a twice-married heterosexual with children. So then I had to increase the age to 50-year-old guys. Jeremy Irons thought the part was too dark, unlike, of course, Claus von Bulow who won him an Oscar. Anyway, Gabriel Byrne absolutely loved it and jumped straight away."
And although Julie Walters, as a widow with designs on Grant's father, and Miranda Richardson, as Grant's mother, were quickly on board, he struggled to cast his American stepmother.
"I tried every American actress in their mid-thirties to mid- forties. You always read about them complaining about how there are no decent parts in their age group - and I thought 'great, they're going to jump at this one'; but oh, no - it was all 'she comes on in page 30; what's the problem?'. And then they heard that the budget was four-and-a-half million pounds. For the whole movie, as opposed to for their client. So, anyway, I'd worked with Emily Watson on Gosford Park - and my wife, who's a dialect coach, suggested her."
Grant has been married to his wife, Joan, since 1986 ("I'm a very loyal person. I think I put a higher value on monogamy because I witnessed the emotional cost of my father's cuckolding") and they have a teenage daughter, Olivia, who has a small role in the film.
Naturally Grant kept a diary of the filming in Swaziland - a mountainous area to the east of the Transvaal, which gained independence from Britain in 1968 and which Grant himself left in 1982. The country is now an effective dictatorship under King Mswati III, where nearly half the population is HIV-positive and life expectancy is 38. "No film had ever been made in Swaziland before," says Grant. "We had purchasing power to power of 12 - it was like making The Lord of the Rings out there." Maybe so, but the ensuing book of his experiences, The Wah-Wah Diaries: the Making of a Film, documents the five-year slog that led to this 100 or so minutes of celluloid.
Grant reckons he learnt the necessary fortitude from Robert Altman. "I love the democratic way he works," he says. "And I like Altman's attitude of just keeping going. When Jude Law pulled out of Gosford Park the financing collapsed, but he just kept on going."
Wah-Wah is a richly enjoyable, sometimes moving and often very funny human story, one of whose positive side effects has been to effect a reunion with Grant's mother, now aged 77 and living in South Africa.
"My mother hasn't yet seen the movie but she has read the script. I was cacking myself when I sent it to her - I thought she may never speak to me again. But she wrote back with her point of view of what it was like living in that sort of society and that time. And it's brought about an understanding - a rapprochement - with her, which was absolutely amazing."
There has been no reunion, however, with Grant's estranged younger brother, Stuart, an accountant who also lives in South Africa. "We never had anything in common," says Grant, who got his revenge in first by keeping Stuart out of the film.
Grant is famously gushing, but there is no doubting his sincerity when he says: "Without a doubt this has been the most intense and creative experience of my life. It was a genuine labour of love - and the opportunity to revisit and recreate your past in the actual locations where it all happened is something which imparted a tangible authenticity to the film."
'Wah-Wah' goes on release on Friday