Perhaps the main reason Gleeson phoned it in for Joffrey (or so he claims) was that he didn't honestly enjoy working on set.
Specifically, Gleeson isn't intrigued by the fake aspects of filming an especially effects-heavy show like Game of Thrones. "When you go on to a set, you realize the very visceral reality that you see on the TV show is a product of a huge amount of processing and editing and storytelling that is not present at all on the day of the filming," he explained to Vulture.
Worse yet, Gleeson found actually doing his job to be almost entirely unfulfilling — and incredibly boring. "You go and put your makeup on, put your costume on, wait in your caravan-trailer thing for an hour and then you go the set, and you film for three minutes and then they change around the lights, and the camera angle and you wait for an hour, and then you film for another three minutes. It's a very boring, tedious process."
Gleeson was so not into it, that his favorite moment of filming actually came when he played dead during Joffrey's funeral, simply because he got to sleep all day. "That was a fun experience," he joked.