American Sniper: How Bradley Cooper Transformed Into Chris Kyle
'American Sniper' has become the most talked-about film of the year so far thanks to it's controversial subject matter and massive box office success in the US.
It stars Bradley Cooper as real life army sniper Chris Kyle - who was the most deadly marksman in US military history - and was directed by Clint Eastwood.
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Cooper has been praised for the way he both physically and emotionally transformed into Kyle - who was shot dead at a shooting range in Texas in 2013 - but how did he do it?
In this exclusive behind-the-scenes video (above), he describes bulking up to play the character, visiting Kyle's real life family and the sense of responsibility that comes with playing an American hero.
'American Sniper' is in UK cinemas now.