While Will Ferrell and Kevin Hart mine comedy from the idea of prepping someone for jail, prison coaching is actually a real profession. ET's Nischelle Turner talked to one of the best in the business -- the man who was called on when Martha Stewart was headed behind bars.

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Before giving advice to Stewart, John "Doc" Fuller served two stints in prison -- one for forgery and 10 years for conspiracy to distribute cocaine. Now a consultant, he's helped countless people navigate through the prison system and even wrote a book, The Ten Prison Commandments.

Fuller addresses his clients' biggest fears head on.

"'Will I be raped? Will I be killed?'" Fuller said are among people's top concerns. Fuller says that those occurrences are unlikely to take place, but his guidance can lower those odds even more.

"Hopefully I can help [them] avoid situations in which that might occur," Fuller said.

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As for Fuller's No. 1 tip for getting through a prison sentence, he offers this: Fly below the radar. Fuller also advises not to stare at other inmates and definitely don't snitch.

"Inmates hate snitches," Fuller said.

In Get Hard, millionaire hedge fund manager James (Will) gets nailed for fraud and sentenced to a do a stretch in San Quentin. The judge gives him 30 days to get his affairs in order, and within that time, James turns to Darnell (Kevin), a hard-working small business owner who has never been to prison, to prep him for life behind bars.

So how does the film's representation of prison coaching stack up against the real thing?

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"All of those things can happen," Fuller said. "But there was that comedian twist to it that only Kevin Hart could pull off."

Get Hard opens today.