John Bradley developed a stammer during his time on 'Game of Thrones'

He said: "I got into this trap with Sam, I set myself traps, because he had a traumatic childhood and suffered a lot - and it manifested through a stammer and a twitch sometimes. There are takes of me where it felt like it would be five minutes where I am just standing with other the actors and I couldn't speak and I just can't get the words out and certain sounds would trip me up.

"In one scene I came in first with Kit Harrington and in the nicest way he said to me 'can you come in a bit quicker?' so I can react a bit quicker and I was like 'I can't, I just can't.'

"As lovely as the crew were, they assumed it was because I had forgotten my lines. It took me a while to get out of it and it went away over time."

The stammer even began to appear in John's day to day life, as well as in "non-Sam acting", and the star says the struggle was down to his "performance anxiety".

Speaking on The Blank podcast, he explained: "Towards the end of that journey, that stammer would turn up in non-Sam acting and I'd do auditions when I'd stammer because it just seemed like the default place that I went to when I acted.

"I think people thought it was part of me but it wasn't, but it was performance anxiety."

Meanwhile, John previously admitted he was too scared to take any props from the show's set as a souvenir.

He said: "I never took anything. I'm so keen to play by the rules. I've got a real phobia of getting into trouble. But I think if there was one thing that I was going to take it'd be the thimble that Sam's mother gave him and comes into play in season two.

"There's a lot of sentimentality tied up in that, it's the last remnant of a loving relationship that he had before he met Gilly (Hannah Murray), and there's so much projected onto it as a relic and what it means to him.

"So I would have taken that but I've not seen it since season three."