Movie star who needs to shoot a door off its hinges? Then Scott Nelson is your man...

You wouldn't know it to look at the footage, but when the square-jawed actor Armie Hammer shot a door clean off its hinges on the set of Clint Eastwood's recent movie J Edgar, he was being given expert coaching by one of America's most accomplished door-shooters.

Scott Nelson, who was standing out of view just a couple of yards away, is a former agent who spent 25 years with the Federal Bureau of Investigation. As a "technical consultant" on the historical movie, his job involved helping Hammer to correctly use a vintage Thompson sub-machine gun to blow apart the woodwork. "Those Thompsons have a hell of a kick," he says. "They can knock you off your feet. So shooting them isn't easy. It has to be done in a stance where you have to crouch and lean. I was standing just out of sight of the camera, helping Armie fire it properly, and also making sure the hot shell casings didn't hit anyone."

Mr Nelson, 69, is part of a now-sprawling community of expert "consultants" who help Hollywood film-makers to realistically portray everything from fight scenes to the intricacies of real-life police investigations. On J Edgar, a biopic of the FBI's founder, J Edgar Hoover, Mr Nelson spent six weeks helping Eastwood recreate the bureau's wood-panelled HQ, and ensuring that cars, jargon, tactics and the demeanour of agents in the film were historically accurate.

"People don't always understand what a real FBI agent looks like," he says. "In my experience, they look purposeful, put together, neat, and very respectable. I met Hoover twice, early in my career, and he could be very demanding about appearances. If you worked for him, you had to have a white shirt, a moderate tie, a blue or black suit and absolutely no facial hair."

Mr Nelson, who later rose to head the FBI's public affairs office, where he helped to launch the television series America's Most Wanted, counts as highlights of his consulting career work on the series 24 and the film The Silence Of The Lambs, where he helped persuade the FBI to allow filming at its

training base in Quantico, Virginia.

There is barely a studio movie in production now that doesn't use a retired boxer or martial artist to help with fight scenes, or a former marine to ensure that battlefields are realistic. Medical dramas are vetted for howlers by medics; university professors cast their eyes over science-fiction scripts. The trend partly reflects a growing need for "special features", such as interviews with consultants, to lift DVD sales. And it also stems from increased studio interest in "real life" stories and historical biopics.

"When you are telling a true-life story, a little inaccuracy provides a foothold for people who want to be critical of the whole project," says Dustin Lance-Black, the screenwriter of J Edgar. "So it of course helps me, as a screenwriter, to really stand behind a project if I can say that we did everything that we could do to get the important facts right."

Black won an Oscar for Milk, a biopic of the pioneering gay-rights activist Harvey Milk, on which he consulted several of his subject's acquaintances.

The rise of the internet has added to the market value of technical consultants, he adds, since armchair viewers use the web to identify perceived historical inaccuracies in films. "When they made a movie like Patton [the biopic of the Second World War general] people would have to go to libraries and look at reference books to check things out."

The job isn't always glamorous. A Los Angeles police officer called Brett Goodkin was suspended from active duty recently after it emerged that he was a technical adviser for Sofia Coppola on The Bling Ring, a film about a group of teenagers who burgled celebrity homes. Prosecutors were, understandably, concerned that the paid gig would undermine his credibility as an impartial witness in the group's imminent trial.

Mr Nelson, meanwhile, had to put up with calls from old FBI colleagues and contacts who were angered by a controversial plotline in J Edgar that suggests the protagonist was a repressed homosexual with a keen interest in cross-dressing. "It's a very sensitive issue which galled a lot of the FBI guys," he says. "But as I told them, I don't have final say on films. In this job, you just advise."

Go-to guys: Celebrated hollywood consultants

Rob Jeremy

The pornographic actor, who has starred in almost 1,300 films, is the go-to-guy for directors seeking to portray the reality of the adult movie business. The 59-year-old Jeremy (pictured above, centre) has consulted on a range of films from 9 Weeks to Boogie Nights – a movie which he claimed portrayed the adult film industry "exactly as it really happens".

Angelo Dundee

The legendary boxing trainer, who shaped the careers of Muhammad Ali and Sugar Ray Leonard, showed a long list of actors – from Will Smith to Russell Crowe – how to box convincingly. "I help actors look, act, and fight like the champions they are going to portray on screen," said Dundee, who died in February at the age of 90. "It's easy as long as they listen."

Dale Dye

A Vietnam War veteran with close ties to Steven Spielberg, the former US Marine captain, 67, is Hollywood's pre-eminent battlefield consultant. His credits as an adviser include Platoon and Saving Private Ryan, and he often appears on screen. He sends actors to boot camps where, he says: "I hurt 'em so hard and took 'em so far out of their comfort zone they became numb and confused."