Anyone who has seen It's Always Sunny in Philadelphia even once knows Charlie Day. His character on the show, also named Charlie, is an unforgettable combination of vulnerability, rage and abject stupidity, and virtually every episode includes at least one or two choice "Charlie moments" in which Day offers an observation, idea or even just a shriek or two that is idiotically, irresistibly hilarious. This week marks Day's first major opportunity since Sunny started to bring his Charlie-style charm to the big screen, as he plays Dan, a predictably dim-witted buddy to Justin Long in the romantic comedy Going the Distance.

Cinematical sat down with Day at the recent Los Angeles press day for Going the Distance. In addition to talking about the shades of difference between Charlie and Dan, Day talked about maintaining a balance between characters that are empathetic and just plain idiotic, and examined the challenges of expanding his repertoire to include a larger variety of roles both in films and on television.

Cinematical: When you come in to play a character like this one, are they sort of offering you the role based on the character you play on It's Always Sunny in Philadelphia, or did they ask you to come in and bring the character to life?

Charlie Day: I was very fortunate in this particular role in that I did not have to audition for it, but I feel as though if I had auditioned, I wouldn't have gotten the part. Because so much of it wound up being improvised, and you can't improvise so much in an audition [because] people just won't know what they're going to get. You can do it on set and they can fire you, but I was willing to hedge my bets. But it was great; Nanette was a fan of It's Always Sunny in Philadelphia, and Geoff, our writer, was a fan of the show, and Mike Disco and Dave Neustadter at New Line, they were big fans of the show, and they came to my office to have a meeting with me at Sunny, which was very different – instead of me going out to the executives. So everything was just kind of lined up for this part, and I wasn't an idiot – as soon as they offered it to me, I knew it was a good thing to take, and I'm really glad I did because I like the movie Nanette made.

Cinematical: I would say that Dan is a marginally smarter, more sensitive version of Charlie – he has to be able to offer these humorous non sequiturs but also deliver some sage wisdom on occasion. How do you approach make sure that these dueling impulses sort of congeal in this character?

Day: You know I think both what I've done in Sunny and on this movie and the movie I'm doing this year, I imagine unless I go full Gary Oldman or something, you're always going to see shades of Charlie just because there's a lot of my own personal sense of humor in how I play those roles. So every time I do a comedy it's going to be hard to do that without my sense of humor. What was nice about this film was that I got to have a few actual heartfelt moments, which there's not that many of in Sunny, obviously – it's a little more cynical. So my only key for doing that or what I feel like I like to see in actors is just to try and find something in me that's the way I would talk to a buddy, or the way I relate. If I was talking to Nanette about how to do a scene, I would propose to her ways I would actually console a friend, knowing that Dan had to be the character that Garrett got some comfort from - having that as a road map. So it was nice to be able to have those colors and at the same time, anytime I wanted to do something outrageously funny, I knew it was going to slip into some [permutation of Charlie] because he's sort of a slovenly, idiotic mess, and so is my character in Sunny. So you get those overlaps, and I'm doing a film now called Horrible Bosses, and at least this character has a job (laughs). He can actually hold down a job – I think he can read and write, and can do math. But, you know, he's still an idiot from time to time, so we'll see. Someone will give me a chance to go Oldman again; it's been a while – not since the Third Watch days.

Cinematical: One of the things you mentioned was that this was a shift for you in that you had to put your trust in someone else that the movie or performance will come out well. Is that trust proven true only when you see the final film, or at what point do you realize that what you're doing is what they want, or is working?

Day: You know, it was easy. On the one hand, Nanette was never pushing myself in a direction, or Jason it seemed, where she was telling us, "you should say the line this way because this is the funny way to do it." That's where my trust really would have been challenged, I think, because comedically you come to trust your own instincts so much that it's hard to get someone else's instincts into your bones – and it's potentially dangerous. But like from a coverage and a shooting standpoint, we're so used to doing it all on Sunny and being so involved in every aspect of the show that it was hard to kind of see her process and trust that it would all cut together and work, and that it was the right way to tell the story. And in that regard, I really pleased to see how well stuff cut together and what a great overall feeling it all had, and I was able to learn a little bit about a different style of shooting and say, you know, you can do it this way and it can still have the feel that we like to have on our show. And even on this movie, it's tough to squash the instincts of producer-writer-actor, and just be actor for hire. But fortunately Nanette and Seth Gordon, who I'm doing [Horrible Bosses] with, they're very collaborative directors, and any smart director is open to collaboration and then draws his or her own lines when they've got to make those final decisions.

Cinematical: I'm a huge fan of It's Always Sunny, and one of the things you guys have been so successful at it maintaining a balance between depicting the most horrible people you could imagine and horrible people that you still find funny and interesting. It might have been less of an issue in Going the Distance, but is that an ongoing process of discovery in maintaining that balance, or having done the show for a while, do you know how far you can go and retain the audience's sympathies?

Day: I think they key is always in the intentions. Our characters in It's Always Sunny in Philadelphia do horrible things, but their intentions are usually never overtly malicious. Maybe they have a scheme or they found a way to make money, maybe they're just trying to one-up one another, and then the outside world gets disturbed and confused by them, but they're never really like, "hey – let's go f*ck with the outside world and ruin someone's life." They kind of accidentally ruin someone's life. So that's the fine line, I think – if you're going to root for the people, it helps to know why they're doing the terrible things, and then you can sympathize, or find humor, in their ridiculous motives. But yeah, it is a fine line. I think when it gets malicious is when it gets unfunny.

Cinematical: Watching you steal scenes in this film, I was reminded of the success of Zach Galiafianakis in The Hangover, which he's been able to capitalize on in other movies. But do you have to be judicious about what roles and what films you choose in order to make sure you don't wear out your audience on one type of character, even if he's been well-received in the past?

Day: For me, yeah, there's a fear - always in comedy, there seems to be a fear of overkilling what you do, although we do always welcome back our favorite comedians once we have a little break from them. But for me if there's a little bit of heart to the character, like what I liked about Dan was how much he cared about Garrett, and I feel like that helps ground some of the ridiculous behavior. Charlie Kelly on It's Always Sunny in Philadelphia is desperately in love with The Waitress, and I feel like he can go to absolute extremes because you can identify with the fact that he's a man who's desperately in love. Even in Horrible Bosses, I play a guy who's so in love with his fiancee that he won't sleep with Jennifer Aniston. So for me, in picking the roles, I always have to find that one really human thread that at least I can relate to so that I can say, that's a real guy, that's a good guy. Or, even if he's a bad guy, he's a real guy. If I read something and it's just, oh, he's wacky and funny, then you run that rick of turning into a cartoon character. But, at the same time, when it comes to choosing roles, no one's offered me the next Batman, so I don't know how much choice I'll get (laughs). But fortunately, because Sunny is so successful that I think I'll have some liberty to wait a little bit.

Cinematical: Having done a few movies and a lot of TV, is there a distinguishing characteristic between the two media in terms of developing or performing?

Day: As a performer?

Cinematical: Or what you do to prepare.

Day: Yeah. I mean, they shoot differently – we shoot differently on It's Always Sunny in Philadelphia, but I think as a performer it's no different. I mean, I want to make evcerything as real or as entertaining as I can, so it really depends on the script and the movie or the television show and just trying to service that story and do it the best that I can do it. That's my only magic trick – I'm just trying to do a good job (laughs).