Six years ago, in the heart of America's Bible Belt, Susan Smith drowned her sons. When Don Boyd set out to document the events for TV, he found that his stereotypes of the Deep South needed revising

Sheriff Howard Wells is a tall, balding man with a moustache and soft-spoken Southern drawl. He reads the Bible and likes to shoot ducks. When he graduated from South Carolina's police academy to become sheriff in Union, handling the media was the last thing on his mind. But after a year in this tiny, economically backward cotton-mill town, Wells was masterminding and manipulating the largest media invasion the state had ever seen.

His sleepy, Capraesque community became infamous as the place where Susan Smith murdered her two young sons, Michael and Alex, by drowning them in the John D Long Lake, a few miles from Main Street and the white, clapboard courthouse that looks as if it has been imported from the backlot of a Hollywood studio.

Smith made an hysterical 911 call to the sheriff's office on the evening of 25 October 1994, announcing that her sons had been abducted by a black man from her car at a traffic light outside Union. There was a massive, frustrating manhunt and an international media frenzy. The boys' pictures were on every front page and on all the TV news shows in the US within 24 hours. There were the inevitable racial slurs - the Deep South's insidious heritage in this arena came into play. Like all Southern cotton towns Union has a black community literally on the other side of the railtrack. Its leaders were immediately pulled into the search and suspects were detained.

But Wells, the antithesis of the stereotype Southern sheriff epitomised by Rod Steiger in Norman Jewison's In The Heat of the Night, smelt a rat. He is godfather to the children of Smith's brother. He pulled Susan Smith in, questioned her in secret locations away from the media scrum besieging his office and exacted this confession: 'I was emotionally distraught - I didn't want to live anymore. I felt I couldn't be a good Mom. I felt I had to end our lives to protect us from any grief or harm. When I was at John D Long Lake I had never felt so scared and unsure as I did then - I was in love with someone but he didn't love me and never would. I wanted to end my life so bad. I was in my car ready to go down that ramp into the water. I dropped to the lowest when I allowed my children to go down that ramp and into the water without me. I took off running and screaming, "Oh God, oh God. No. What have I done? Why did you let this happen?" '

Smith had pushed her burgundy Mazda into the water with the children of her broken marriage to David Smith strapped in the back - screaming as the water engulfed the car. The sheriff's divers had missed the car in the murk on their first search. Within hours of the confession, they returned and dragged the boys' waterlogged coffin out of the lake...

My image of the Deep South has been moulded by the plays of Tennessee Williams, by great Hollywood movies like Gone with the Wind and In the Heat of the Night, and more recently by films like The Big Easy and Mississippi Burning. Hot, steamy nights full of passion, hypocrisy, neuroses and racial prejudice.

I had two early experiences in the South. I had produced a documentary in 1979 about a spitting competition in Raleigh, Mississippi - the people I stayed with were special, God-fearing and generous, although the armoury of more than 20 guns under the staircase, the ammunition-manufacturing hut and their pet snakes reinforced my view that Southerners were eccentric and idiosyncratic to say the least. And then I had produced John Schlesinger's comedy Honky Tonk Freeway in Florida. Oddly enough we had scouted John D Long Lake for our pink-elephant-on-water-skis shot in that film.

I knew from both those experiences that getting the Union community to talk about the Susan Smith case was going to be difficult after the horrors of the crime, Smith's trial and the consequent media invasion. I decided the only viable ploy was to turn up unannounced and proclaim my mission. I arrived in Union on the five-year anniversary of the murders and went to the lake. The golden leaves of fall mixed with pink reflections on the water to leave a blood-red tinge.

I went into Main Street where the town was having a pre-Thanksgiving do. I asked people about Smith - I needed to feel their mood. I was sent packing. Nobody wanted to think about the subject. Especially when quizzed by somebody wanting to make a film about it.

In South Carolina the law forbids filmed prison interviews so Susan would never be available for interview. I wrote to her mother from the Jamieson Inn, our fake clapboard motel. She turned us down flat. I felt I had only one strike left. What I needed was a pillar of Union society whose opinion of me and my mission would be positive enough to garner respect from the other protagonists.

I wanted to talk to and film David Smith, the prosecution counsel, the defence counsel, the reverend at the heart of the racial controversy, the local disc-jockey involved in the search, the woman whose house Susan went to after the murders and, crucially, the sheriff.

My team and I were beginning to get desperate. Would I have to abandon this film? Getting to the heart of a society which throws up a mother who kills her children was proving difficult. I decided to have cocktails in Union's most notorious bar, Smith's last port of call before her murderous drive. It had changed its name and was empty apart from its new owner, Wanda, who was sweet but hadn't been in Union at the time of the murders. A drunk who had known Smith came in and shouted abuse at me when I offered him a beer. The evening was a disaster. Nothing.

But like most of America, and particularly the Southern states, what you see and what you associate with Hollywood is not necessarily what you get. Linda, who owns the Fairview Diner, persuaded me to talk to Sheriff Wells first. She said I had the wrong image of the Deep South. For example, Wells - a candidate in my mind for the Ray-Ban sunglasses and gum-chewing violence of the small-town sheriff in In The Heat of the Night - was in reality the antithesis of Rod Steiger's self-serving bigot. Although the sheriff's office turned us down when approached from London, she thought it was worth paying Wells a visit.

The sheriff's testimony had been meticulously professional without being biased. He had taken the dovish view, as opposed to his opposite number, Tommy Pope, who had been relentlessly hawkish - Pope had wanted Smith to be executed. But it was difficult to tell what side Wells was on from his testimony.

The sheriff's office is next to the room used for Union's last execution some 80 years ago. But Wells's first proclamation to us was clear - Union was anti-death penalty. Duck pictures hung on the wall. A picture of the President, his family and the obligatory Bible. The rest of it could have been a corporate vice-president's working space. I realised I had to be precise and direct. Wells would see through any flannel.

Wells talked about racism. He had been embarrassed by Smith's clumsy attempt to deflect her guilt towards an imaginary black abductor. He had known she was lying. 'Susan almost confessed on the second day,' he recalled. 'The first time she was polygraphed by the FBI in this very building, Susan and I had a conversation after the test was concluded - no one else was in the room - and she asked if I would pray with her. She put her hand in mine and we prayed and she broke down and she cried then, and she later told me after everything was done that she just about confessed on the second day but didn't know how.'

As we talked more I began to warm to this man. He had argued vociferously that Smith be tried within the local community. He felt Union citizens were more likely to give her a fair trial, knowing her way of life and her state of mind. Smith was abused by her stepfather. Her father had committed suicide. She herself had made two failed suicide attempts. She was divorced before her twenty-third birthday. I could see Wells struggling, even five years after the crime, with his moral dilemma: How could any Christian, armed with all these facts, come to the conclusion that the aunt of his godchildren had wilfully murdered her little boys for the sake of a doomed love affair?

Far from conforming to the clichéd view of the Southern Sheriff (who, in his own mocking words, 'chews tobacco and says, "Boy, you're in a heap of trouble"'), Wells was savvy enough to use the media as part of his strategy. 'The whole thing was to keep [Smith] involved, allowing her access to the media, so everyone could see, and it was interesting that she would respond not directly to us but through the media to what we'd asked that day.'

I told Wells I had had huge sympathies for Smith. I said we would not be able to make a good film without his support. But I warned him I would have to present the prosecution case. There are many people who think Smith should have been executed.

We left with his tacit blessing and a promise to see us when we returned to shoot - a promise he honoured with an articulate and sensitive interview. His endorsement opened up a vista of contributors, including David Smith, and allowed me to make my film with authority. And my encounter with Sheriff Wells brought me up to date with the evolving psyche of the American South.

• Women Who Kill, ITV, 9pm on Thursday

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