He said he had "re-evaluated" what he wanted to do after studying philosophy and theology at Dublin's Trinity College.
"It was always a recreation beforehand, but when I started doing Game of Thrones, perhaps the reality was made too real for me.
"The lifestyle that comes with being an actor in a successful TV show isn't something I gravitate towards."
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.
- nzherald.co.nz