Being a Hollywood star is often considered to be a relatively easy job.

You learn your lines then go to work and repeat them with emotion in front a camera ... Sounds easy right?

But it's a heck of a lot tougher than that and it can actually be incredibly dangerous.

Even though CGI nowadays is so realistic, actors and actors are increasingly going above and beyond to complete their own stunts and some of them are absolutely terrifying.

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Margot Robbie in Suicide Squad

One of Robbie's fight scenes in the box office smash (that's been panned by critics) took place completely underwater and required rigorous training with a free diver.

Margot Robbie stars as Harley Quinn in Suicide Squad.
Margot Robbie stars as Harley Quinn in Suicide Squad.

During a recent appearance on The Tonight Show Starring Jimmy Fallon, the Aussie actor joked about her fierce competition with her stunt double, Ingrid.

"I could do a resting breath hold of five minutes ... then [Ingrid and I] started getting really competitive," Robbie said.

"She would do a session and was like, 'I made it to three minutes' ... then I made it to three and a half ... four ... then I made it to five and was like this is above and beyond what I thought I could do, I'm good.

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"She did one extra session and was like, 'I got to five and a half'."
After that Robbie insisted on one more session but was shut down.

"No one needs to drown over this," her superiors told her, "The scene takes one minute; we don't need to do this".

Robbie laughed at herself in the end because despite fighting underwater for a number of minutes, they only used a small part of the fight in the final edit.

But she wants everyone to know, "I was under the water for so much longer than that!"

Isla Fisher in Now You See Me

Fisher played a magician in the 2013 movie and was lucky to survive after a stunt went wrong. The Aussie star almost drowned when she became stuck inside a water tank.

Her chains became stuck and Fisher was unable to reach the kill switch which would've drained the tank instantly.

Isla Fisher did all her own stunts in the film Now You See Me.
Isla Fisher did all her own stunts in the film Now You See Me.

"They had a guy who was off camera, but he was a long way away with a can of oxygen," she said.

"By the time I realised I couldn't get up and beckoned for him, I realised that I had run out of air."

Fisher was underwater for two and a half minutes. Her direction for the scene was to bang on the tank glass and pretend like she was drowning.

It's safe to say she gave a pretty realistic performance.

Kit Harington in Game of Thrones

Game of Thrones

has received critical acclaim for the blockbuster quality of their TV show and last season's episode nine on the battle for Winterfell was no different.

Harington starred in a scene where his character, Jon Snow, is faced with a cavalry wall of forty horses charging at him full speed. It was all real.

Kit Harington plays Jon Snow in a scene from Game of Thrones.
Kit Harington plays Jon Snow in a scene from Game of Thrones.

"Up until the last minute I was stood there facing off against this cavalry charge which is really scary," he said.

"We're a bit annoyed because I think everyone is going to think it's CGI, but it wasn't," he laughed.

Game of Throne's dedicated Horse Master, Camilla Naprous, pulled off the iconic scene by making a series of channels for the horses to pass through that were each only two feet wide.

And how much did they spend on the episode that needed 600 crew members, 500 extras, 70 horses and 25 days to shoot? Upwards of $10 million.

Tom Cruise in Mission Impossible: Ghost Protocol

The question of whether Tom Cruise is partially insane often comes up from time to time, no more so than when people discover the outrageous stunts he's attempted in the past.

He once even fired an insurance company who were looking after the fifth Mission: Impossible movie purely because they wouldn't let him attempt a stunt.

Tom Cruise in Mission Impossible Ghost Protocol.
Tom Cruise in Mission Impossible Ghost Protocol.

Once they were gone he gave the stunt a crack where he hung off the side of the world's tallest building, the Burj Khalifa in Dubai.

The stunt took Cruise's career, and his willingness to perform the most insane of stunts, to new heights ... literally.

Also worth an honourable mention is Cruise's stunt in Mission: Impossible-Rouge Nation when he clung to the side of a plane after it took off.

He was protected by just a small safety harness which was later digitally removed after Cruise shot the scene a ridiculous eight times.

Frank Grillo and Scarlett Johansson in Captain America: The Winter Soldier

Grillo and Johansson made a number of death-defying jumps from a crane which was 60m high.

Grillo originally refused to do it but was forced to give it a crack when Johansson teased him.

Scarlett Johansson plays Black Widow in The Avengers movies.
Scarlett Johansson plays Black Widow in The Avengers movies.

He told Esquire, "We were filming the Captain America sequel and we were about 100 feet off the water on these big ships. They put me on a crane that took me higher - about 200 feet up - and they literally dropped me in a free fall. Halfway down they pulled me to make it look like I was landing on a parachute".

"I was like, 'I'm not doing that.' Scarlett Johansson then gets up there and is like, 'What are you not doing?' I said, 'They want me to free fall.' She said, 'I'm going to do it right after you.' So now I had to do it. I had to do it about 10 times.

"There were parts of my anatomy that were up in my throat by the time I was finished. But I did it. And of course she did it with no problem."