EXCLUSIVE: How Whitey Bulger’s henchman was so convinced Johnny Depp was actually the mob boss he almost punched actor: Gangster speaks as Depp runs for Best Actor award this weekend

As a hardened gangster not much ever fazed John 'Red' Shea.

Having been a key member of James 'Whitey' Bulger's brutal gang he had seen and done it all.

But when he saw Johnny Depp sitting in his makeup chair having been transformed into the feared Boston mob boss for the film 'Black Mass' his heart almost skipped a beat.

Having once been one of Bulger's most loyal lieutenants and enforcers Shea now had nothing but hatred for the man he once idolized.

Like many in the Bulger gang he was shocked to learn his former boss had been an FBI informant.

While Shea refused to betray the code of silence held by most mobsters Bulger had been happy to 'rat out' his friends.

'I had not seen Whitey in almost 20 years and suddenly he is sitting in front of me,' John tells Daily Mail Online in an exclusive interview.

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'Reunion': 'I could feel all the emotions building up inside me and had to leave the trailer. If I had stayed there I would have hit Johnny,' says John 'Red' Shea who was a key member of Whitey Bulger's brutal gang. He met Johnny Depp as the star played Bulger and was astonished at the likeness

Uncanny: Johnny Depp (right) is nominated for Best Actor at the Screen Actors Guild Awards for his portrayal as Boston mob boss James 'Whitey' Bulger (left)

Friendship: Shea and Depp became close friends after meeting on the set of Black Mass. Shea was able to give Depp a chilling insight into what life was like working for the man who became one of America's most notorious gangsters

'I knew it was Johnny Depp, but I had this urge to hit him. I could feel all the emotions building up inside me and had to leave the trailer.

'If I had stayed there I would have hit Johnny.

'Outside I was just so angry, I was shaking and felt like punching the trailer. That's how convincing Johnny was. He was Whitey.'

Fortunately Shea was able to calm down and he and Depp, who has won rave reviews for his chilling portrayal of Bulger, became close friends.

Shea was able to tell Depp all about his life with Bulger and give him a chilling insight into what life was like working for the man who became one of America's most notorious gangsters.

Depp is nominated for Best Actor at the Screen Actors Guild Awards on Sunday 30 January and is also tipped for an Oscar nomination.

The 52-year-old actor's transformation into Bulger is down to upper half of his face being fitted with a prosthetic and the creation of 50 different headpieces to mimic the real life Bulger's receding hairline.

So convincing was the work of Oscar winning makeup artist Joel Harlow that when Shea first saw Depp he was convinced he was once again in the presence of the murderous mob boss.

Shea, 50, had been his protégé and helped him run a vast drug smuggling operation in the 1980s that made millions.

Like Bulger he had grown up in 'Southie', the then-rundown area of Boston, where Bulger built his empire in the 1970s and 1980s.

He was able to avoid arrest thanks to a pact with a corrupt FBI agent called John Connolly.

In return for information about his Italian mafia rivals Bulger was left to do whatever he wanted - and that included killing as many as 21 people.

Southie born and bred: Shea was raised in South Boston and got caught up in Bulger's gang. In this childhood photo Shea (right in striped suit) is being given a trophy

Hands up: Shea earned the nickname 'Red' because of his ginger hair. He took up boxing as a young child and posed with other young boxers (bottom right)

Home: Shea grew up in 'Southie', the rundown area of Boston, where Bulger also grew up and built his empire in the 1970s and 1980s. It is now more up and coming 

Vicious boss: 'Once he lost his temper, then mercy on the person he was angry with. They would be lucky to get out alive,' reveals Shea of Whitey's true colors

Victims were smashed in the skull with an ice pick, strangled, blasted to death with a shotgun or shot, execution style, in the back of the head.

Among the victims were Paul McGonagle, a rival gang member, and Edward Connors, who was killed because he had witnessed another killing Bulger's gang feared he would talk.

Others murdered included businessman Roger Wheeler and Brian Halloran an FBI informant who died in a hail of gunfire as he left a South Boston restaurant.

Boston police were desperate to bring him to justice but every time they tried to build a case against Bulger the FBI would shut down the case.

Shea played no part in the murders but was responsible for bringing millions of dollars of worth of drugs from Florida to Boston.

He reveled in his life as one of Bulger's most trusted men.

But despite their closeness he knew Bulger could suddenly turn and lash out.

'Whitey was the king of kings, the Irish Godfather and one of the most dangerous and cunning guys around.

'He could be very charismatic and a great friend. He taught me a lot but he could also be very vicious.

'Once he lost his temper, then mercy on the person he was angry with. They would be lucky to get out alive.

'Whitey had such a fearsome reputation that people were terrified just being around him.

'He had got to the top by knowing who to befriend and who to betray.'

Like other members of the Winter Hill gang Shea had no idea Bulger was an FBI informant.

In 1990, he was arrested on drugs charges and while offered a plea deal to 'rat' on his boss Shea chose to stay silent.

'I wasn't going to be a rat bastard,' says Shea.

'It was my duty to protect Whitey and I stayed silent. I knew others were considering deals but it was important to me to say nothing. I had been brought up knowing that anyone who rats out to the police is the lowest of the low. I wasn't going to do that. Not to Whitey.'

Shea served 12 years after being convicted of drug smuggling.

While serving his sentence he learned Bulger had gone on the run after being exposed as a FBI informant.

Johnny Depp and Joel Edgerton in Black Mass. Edgerton played John Connolly, former FBI agent who was convicted of several charges including murder from his relationship with Whitey Bulger

Shea was also an advisor on the film 'The Departed', which was based on the life of Bulger. It starred Leonardo Di Caprio, Mark Wahlberg, Matt Damon along with Jack Nicholson as Bulger

'While I was in prison there were rumors that Whitey was a rat. I didn't believe them and would have killed anyone who said it. I might have been a criminal but I had a code, and that was a code of silence.

'But when Whitey disappeared and went on the run it all came out that he had been an FBI informant for years.

'It was like being hit with a sledgehammer,' recalls Shea.

'For a long time I did not want to believe it.,' adds Shea, whose book 'Rat Bastard' details his life growing up in Boston and joining Bulger's gang.

'Whitey always used to say he hated rats. He taught me that there was no one lower than a rat. That was what I believed, and now I was hearing the man I looked up to was the biggest rat of all.'

As Shea served out his sentence Bulger and his girlfriend Catherine Greig became fugitives.

Shamed FBI bosses were now desperate to find their former informant, with Whitey listed as the second most wanted man in America behind Osama Bin Laden.

Shea, who gained the nickname 'Red' because of his ginger hair, was convinced his old boss would never be caught.

He knew Bulger had stockpiled cash and calculated he would now spend it on plastic surgery to disguise his identity or to set up home far away from the US.

But he was stunned when Bulger and his girlfriend were found in 2011, living in a flat in Santa Monica, California.

The FBI knew Greig, who was 22 years Bulger's junior, enjoyed beauty treatments.

They ran a series of TV adverts with computer generated photos of how the by-then 81-year-old Bulger and Greig, 64, might look.

They also mentioned that Greig was thought to love visiting beauty salons.

Whitey Bulger and his girlfriend Catherine Greig became fugitives in 1995 as law enforcement was about to arrest him. 

Bulger and Greig were found in 2011 living in Santa Monica after Greig was recognized by a former Miss Iceland, who called authorities. Bulger gave up without a fight and after 16 years on the run

Greig was recognized by a former Miss Iceland living in Santa Monica, who called authorities.

Bulger gave up without a fight and after 16 years on the run was sent back to Boston to stand trial.

Shea was asked to give evidence by prosecutors but refused.

Even though he had been betrayed by his old boss he wasn't about to break his code of silence and become an informant.

'They had me listed, but I told my lawyer no way will I say anything. Even though Whitey was a rat, I wasn't about to become one,' says Shea.

In August 2013, after an emotional packed three month trial, Bulger, now 86, was found guilty of 11 murders.

He was sentenced to two life sentences and will die in jail.

Among those who had followed the trial was Patricia Donahue, 70, whose husband Michael was shot dead by Bulger in 1982.

While many of Bulger's victims were criminals, several were innocently caught in the crossfire, such as truck driver Michael.

The hard-working dad of three young sons was simply in the wrong place at the wrong time.

He had offered a lift to his friend Brian Halloran, who had told the FBI about a murder Bulger had committed.

Bulger was told about Halloran by his corrupt FBI handler and ambushed him - with Michael being murdered simply because he was in the same car.

Patricia made sure she was in court when her husband's killer finally faced justice more than 30 years after the murder.

She tells Daily Mail Online: 'I wanted to look at the man who had changed my whole life in a blink of an eye.

'I had nothing but contempt for the man and hated him for what he did. He looked pathetic sitting there. He would not even face me.

'I have learned to live with what happened but my sons despise him and even after all these years are still full of anger. They hate him,' says Donahue.

Once the king of Boston, Bulger is now an inmate at the US Penitentiary Coleman II in Sumterville, Florida.

Since leaving prison Shea has rebuilt his life and works in construction. He tells Daily Mail Online he was happy to be on the set on 'Black Mass' with Depp and the other cast.

He was also an advisor on the 'The Departed' which was based on the life of Bulger and starred Leonardo Di Caprio, Mark Wahlberg, Matt Damon along with Jack Nicholson as Whitey. 

Shea hopes Depp wins an award for his portrayal of Bulger.

'I don't think any other actor could have taken on the role,' says Shea.

'Johnny was just unbelievable and he really gets across what Whitey was like. There is menace about him and that's what Whitey was like.'