Nikolaj Coster-Waldau found meeting former prisoners while shooting 'Shot Caller' ''very exciting''.

The 47-year-old actor stars as Jacob Harlon in the new crime thriller by Ric Roman Waugh and he loved doing the research around the prison system and the gang culture.

Speaking to the Metro newspaper, he said: ''One of the things I love about my job is that you get to educate yourself and you get to be exposed to things you haven't before.

''I learned so much about [the prison] system. Once you get institutionalised, how that affects you... to actually meet former prisoners was very challenging and very exciting.

''Everyone looks so tough and mean but when you talk to them they tell you they have to do that. Everyone is s**t-scared all the time.''

The new film also stars Lake Bell, Benjamin Bratt and Jon Bernthal, and follows Jacob, who has recently been released from prison and finds himself forced by his gang leaders to orchestrate a major crime with a rival gang.

While filming, the actor met a former shot caller who had been involved in researching the plot, and the 'Game of Thrones' star learned from him that people in prison don't always commit a violent crime but their behaviour escalates because of the inmates that surround them.

He said: ''The guy who did the research for the film had been a shot caller himself.

''He went into prison when he was 20 and hadn't done any violent crime. He'd done a lot of carjacking and stuff.

''Suddenly you come into a world where you are with very violent people. He raised himself up the ranks and he ended up being a shot caller.

''That was what was interesting about my character in the film. He made a really horrible, stupid mistake - he had a glass of wine too many and drove.

''It was such a stupid thing to do and it had horrible consequences but these things can happen to anyone.

''What's interesting about the story is that it shows you don't necessarily enter the world of prison as someone committing a horrible crime.''