Talk to Day for a few minutes, and you can immediately see how much invention and amplitude go into dopey Charlie Kelly, his TV alter ego.
"Sunny," now filming its seventh season for FX, has a bleakly funny tone and cult status but some of the same hallmarks as classic sitcoms such as "Seinfeld," and "Cheers" - that is, the writers never ruin the characters by improving them as human beings.
The "Sunny" folks don't evolve, they exhibit little moral progress. They retain their foibles and imperfections and weaknesses (in "Sunny's" case, the pathologies) that drew audiences to them.
"You don't want the core of who the character is as a person to change," said Day. "When a character makes a choice, you want to be able to say 'that choice is quintessential Charlie.' "
That continuity has drawbacks, especially in a show like the down-market "Sunny" - change is so anathema that characters don't even change clothes.
"After four seasons, we're like, we need to go to wardrobe and tell them to buy some new jeans. I wanted to at least wear a different T-shirt. And I'm still wearing the same jacket."
The trick, he said, is to change it up without changing the essence.
"You have to try different things. For example, take Rob McElhenney this season. He's gained almost 50 pounds for the character. He's pulled a Robert De Niro."
What kind of 50 pounds? A weight room 50 pounds, or a taproom 50 pounds?
"It's not a flattering 50 pounds," Day said. "He did it in about two months, with a lot of protein shakes and ice cream. He went to a doctor and said, how can I do this without hurting myself."
So he's himself, only more so.
"It's funny, he's still the same character. He still thinks he's an Adonis, and thinks he looks great. That's what makes it so funny."
On Collider.com, the fattened McElhenney talked about how his weight gain complements the show's ideology.
"I was watching this very popular sitcom and noticing about how the characters got better looking as the seasons progressed. And I have never seen a sitcom in which the actors got worse-looking, which is, I think, truer to life, especially the lives that these characters lead."
McElhenney also mentions that the cast this season goes to the Jersey Shore. Funnel cake for Mac?
"Sunny" has made its cast/crew famous and employed - Day is doing "Horrible Bosses," Glenn Howerton has a small but vividly good role in Will Ferrell's "Everything Must Go."
In "Bosses," Day plays a dental hygienist sexually harassed by Aniston, his boss. Aniston is frequently half naked, and Day's character is comically anxious.
"It was easy to play into that fear," said Day, of playing opposite his famous co-star. "I didn't want to do anything that would get me fired."
While "Sunny" has given Day and Howerton a chance to do movies, its time-consuming schedule also limits their opportunities.
"It's a challenge for us, it really is. Rob wants to direct a movie, and that would take at least a year," he said. "When we finish up the seventh season, if we decide to move forward, I think we'll try to find a way to alter the shooting schedule, so there's more flexibility."