Nobody really knew Nikolaj before GoT. Of course he was a looker and had acted in some Danish films. He made his Hollywood debut with the film Black Hawk Down but he only got serious mainstream attention after appearing as the Lannister warrior Jaime, who is involved in an incestuous relationship with his sister.
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Lena was perhaps one of the most successful names to have been cast for the show. She had quite a fan base even before GoT. Not only has she played Sarah Connor in Terminator movies but had also acted in many British films. But her biggest claim to fame is, of course, playing the cruel Cersei Lannister on GoT.
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Alfie who plays the unfortunate and cowardly Theon Greyjoy on GoT had some minor roles as a child and teen artiste in some major Hollywood movies like Elizabeth and Agent Cody Banks 2. But he never really was famous until he starred on GoT as the shady Greyjoy sibling.
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In spite of being a total stunner and popular name in the British showbiz industry, Natalie only got real recognition after starring on Game of Thrones as the manipulative Margaery Tyrell. Though she had starred in the film Casanova and the hit show The Tudors, people around the world only associate her with GoT.
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In spite of being a fantastic actor and a critical success, Aiden never found widespread fame, that is until he starred on GoT. His stint as the evil and plotting Little Finger brought him much acclaim and he even got bigger roles post GoT.
Image Source: metro.co.uk