Gleeson, who first auditioned for the role at age 17, could find no redeeming qualities in King Joffrey, an evil ruler of House Baratheon who ordered the beheading of King Eddard Stark.
"I would like to try and defend him, but I would have a pretty hard job doing it. I suppose he's the product of his context and his family.
"In terms of redeemable qualities, there wouldn't be many."
The fourth season of Game of Thrones is due to air in April, 2014.