A film that Duvall wrote, directed, financed and performed in (winning him his fifth Academy nomination of his career), it has been a project gestating for over 15 years. But, as he explained, the attraction went back even further. "I had seen a preacher 30 years ago in a small church in Arkansas, I was always fascinated in that manifestation. They say the true American art form is the American preacher. Both black and white. The style they preach, they're so alive, so great on their feet. Clinton reminds me of a Pentecostal preacher. He's from Arkansas, could stand in front of a black congregation and speak for an hour. To try and catch that style was interesting to me."
The man frequently dubbed the "American Olivier", known for his ability to immerse himself fully into a role, more than just captures the mannerisms. Duvall's Sonny, who baptises himself the eponymous Apostle, in order to gather together a new congregation in a makeshift church, is a remarkable creation.
With support generated for The Apostle by playwright and screenwriter Horton Foote Jnr, Duvall's Sonny is a direct descendant from the pair's previous work together.
While Francis Ford Coppola may have directed Duvall in his most iconic parts (Kilgore and The Godfather's Tom Hagen), Foote Jnr has provided Duvall with some of his most significant, if not best-recalled, roles. After a stint on stage in New York where he won an Obie for his role as longshoreman Eddie in Arthur Miller's A View From A Bridge, Duvall played the pivotal part of the retarded Arthur "Boo" Radley in Foote Jnr's screenplay of Harper Lee's To Kill A Mockingbird.
Their next collaboration, the little-seen Tomorrow, Duvall still rates as his best work, in which he played a Mississippi farmer, with an accent so convincing even the locals thought he was one of them. While 1991's Convicts, based on Foote Jnr's play about a Louisiana sugar plantation owner, was uneven (despite good work from Duvall), it was Bruce Beresford's Tender Mercies, written and produced by Foote Jnr, that won Duvall his only Oscar in 1983. A self-destructive Country and Western singer who is baptised by his new wife, the part, for which Duvall spent months listening to tapes of the Texan accent, is a precursor to Sonny. Duvall has played a hard-nosed TV executive in Network, an Orwellian techie in THX-1138, Dr Watson in The Seven Per-Cent Solution, Jesse James in The Great Northfield Minnesota Raid and the puritanical doctor in M*A*S*H.
He has acted alongside John Wayne in True Grit and Marlon Brando, who he admits intimidated him, in The Chase. But in his role as Sonny, one instantly forgets the show-reel. He simply becomes the man. Inspired by the naturalism of Ken Loach's work (a lifelong fan, Duvall championed the director in America after Kes came out), The Apostle also draws from a tradition of Southern literature, in which the charismatic stranger heads for a small town to transform the community. Citing works by Faulkner, Carson McCullers and Flannery O'Connor's Wise Blood, Duvall saw Ned Beatty's street-corner evangelist in John Huston's adaptation of the latter as the only faithful on-screen rendering of the profession he had seen. "Films tend to caricature preachers," he notes. "It would've been too easy to portray Sonny as a violent, unlikeable or corrupt man." Thoroughly aware of the extremes religion can inspire - "from Mother Theresa to paedophilic preachers in Southern Ireland" - Duvall's Sonny, both in writing and execution, is a maelstrom of contradiction, dealing with the guilt for his crime.
"Sonny always had redemption in him," says the 69-year-old Duvall, frequently noted as reserved with the press, but on this occasion forthcoming. "He does not suddenly just see the light. It's on-going. He's human. He commits a crime, but he's not as bad as King David in The Bible who sent a man to die deviously. Sonny would never do that. He accepts that he has to pay the secular price." Duvall hawked the project to every major studio and independent outfit, only to be faced with indifference for a script that ran against Hollywood convention. Paying for the $5m budget himself through work he calls "just jobs" - The Scarlet Letter, Phenomenon and the recent asteroid-disaster flick Deep Impact spring to mind - Duvall has a habit of directing un-supported films. Over two decades ago, his debut - a documentary about a Nebraskan Rodeo family called We're Not the Jet Set - received critical acclaim, but disappeared rapidly.
Angelo, My Love, a film he funded himself with $1m six years later, went much the same way. Using, Loach-like, mainly non-actors, it was an anthropological study of the life of an 11-year-old gypsy boy, displaying a quest for truth in much the same manner as The Apostle.
A Christian Scientist himself, Duvall is quiet on his faith: "I have my own beliefs. I'm from a Protestant background. I believe in God and Jesus Christ. Believers probably think I'm going to Hell for this." A friend of his third wife, Sharon, who suffered from cervical cancer at the time he discovered she was having an affair with their pool cleaner three years ago, blamed their divorce (also his third) partially on his religion. "Her illness went against his beliefs that prayer cures sickness," they said.
But Duvall's concern for The Apostle went beyond his faith. "I hoped it would be accepted by the secular and religious communities. Billy Graham called it `a poem for the 21st century'. Whatever acclaim we get is because we did it the way we wanted to do it. For many years I was afraid of the project, but once we started I found it pretty harmonious."
His thinning blond hair apart, he shows little sign of ageing. Sprightly, he talks of taunting Francis Ford Coppola with his mother's recipe for Maryland crab-cakes; of riding pillion with flatmate Dustin Hoffman on a motor-scooter to meet Peter Fonda in the early Sixties when he hung out in New York with unknowns Gene Hackman and Jon Voight; of filming his favourite scene in Godfather Part II (when he tells Frank Pentangeli to kill himself in prison), favoured because actor Michael Vincente Gazzo was drunk all day. Obsessed by food, horses and the tango (he made a film based on the subject, while his ex was the only American who danced professionally with Tango Argentino when they toured), Duvall is gearing up to indulge in yet another passion, football. Having worked on another "job", the forthcoming A Civil Nation with John Travolta, he is currently developing a script about 1960s Scottish footballer "Wee" Jimmy Johnston - "a great dribbler" he claims, an antidote it seems to his dislike for limeys.
Admitting to being an admirer of the current crop of young Hollywood beaus (he cites Sean Penn, Gary Oldman and Johnny Depp), Duvall's speech infers that he pays more attention to his craft than the Hollywood shenanigans. "What that kid did in Gilbert Grape - that DiCaprio - is unbelievable. Kim Stanley once said in America there are a lot of great actors between the ages of 25 and 40, and after that something happens. I was kind of a late bloomer, so I figure I've got a lot to do."
The Apostle opens today