TORONTO - There is something very, very wrong with Charlie Day's eye.

His left eye appears to be filled with blood after every capillary in it burst, and it makes it hard to sit across from him on the set of "Pacific Rim," amidst the smashed and ruined remains of a street in downtown Hong Kong. From where we sit, we can see a hole in the street that was created by a rampaging kaiju that was searching for Dr. Newt Geiszler. Why? Well, it might have something to do with that eye.

"Every time we do something, I go back and look it in the monitors. It's very cinematic in nature and you add that to his imagination… I mean, technically he's a really, really good director.  So then you take his love for his creations and the amazing art departments and all that, and it usually makes for something that's visually just stunning."

He's speaking, of course, about Guillermo Del Toro, the director of this insane film about a world where giant robots and giant monsters do battle over the future of mankind. Day is not a guy you would automatically expect to find on the set of a giant science-fiction action movie if you're familiar with his work, but Del Toro was inspired to hire him after seeing an episode of the sitcom "It's Always Sunny In Philadelphia" where Day did battle with a particularly determined rat. The mania and the commitment that Del Toro saw in that episode was exactly what he was looking for in this character.

"I am Dr. Newton Geiszler. He is probably the foremost expert in the world on the kaiju, and he goes on a journey to get one of the brains of the monsters. As you can probably see, it doesn't go so well for him. But he does get at least enough of what he needs to help the good guys do some fighting."

Watching Day on-set, everything he was doing was very physical. He was trying to scramble away from something that we couldn't see, something that wasn't there on-set with us, and Day threw himself into each of the takes whole-heartedly. It looked exhausting, and we asked him if the physical demands had been more or less difficult than the giant chunks of exposition that his character has to deliver in the film. "The exposition part was fine. I've always been good at talking. The stunts part has been a little more physical than I had anticipated. I'm in a lot of pain, but it's fun.  I mean, it's really fun to have the character sort of start on his high horse and he's a little bit pompous and he's an ass, then literally he's dragging himself into the rocks and the dirt here. It's fun to kind of go on that journey. I've been getting some pats on the back from the stuntmen, but I don’t think there's really been an actor or character on this movie who hasn't been either in pouring rain or strapped to some kind of crazy wires. Guillermo is as talented as he is sadistic."

He went on to talk about why he was excited to do a film like this. "It was one of those things where I sort of hoped maybe if I'm lucky enough to be in a hundred comedies then someone will maybe one day be like, 'Hey, maybe he can be in something else.' Fortunately, Guillermo was just such a big of 'It's Always Sunny in Philadelphia' that he was able to picture me as the character who has to be a bit puckish. I was dying to do it. I've always wanted to do something of this scale. Growing up, I loved 'Indiana Jones' and 'Ghostbusters' and those big, big movies. It's fun to be in something big."

We mentioned to him that Ron Perlman had said almost all of his scenes in the film were opposite Charlie, and asked him how Ron is as an acting partner. "He's great. Ron's a ham in real life, you know, and he carries himself like he was in Sinatra's Rat Pack. It's really been good aside from the amount of spittle I've got on my face 'cause he wears this gold grill in his teeth and it does not hold things in. It does provide a good windshield for spit."

Day cautioned us that while there is certainly an eccentric sense of humor to certain parts of the film, it is not a comedy. "I think anything too comedic would not fit in the tone of the overall movie. Maybe there's a quicker wit to the character than most people, although right now I've hit the dirt so many times this week that I'm talking like Joe Frasier. I think he's just human in the way that funny characters can be some times. He's not the guy with the abs who can jump into a robot and punch a monster out. He is sort of the man on the street. He provides moments of levity, but then there's some really wonderful moments of true fear and horror and heartfelt intention. I'm glad he's letting me do it."

One of the gathered reporters asked Day if he's the real hero of the movie instead of the guys in the giant robots, and Day smiled broadly, taking a moment before he answered. "Are you trying to get me in some hot water here?" He laughed before he went on. "No, it works hand in hand, just like it works hand in hand in real life. It's true… us nerds do save the day a lot of the time. Those tough guys do some good fighting for us, too, so it really is a fun ensemble film in that way. You're not just watching one hero go through. All the characters get their opportunities to save the day in different ways. My guy just does it in a way that's maybe a little bit more relatable to the average viewer, and I think it's the filmgoer's chance to kind of put themselves in the shoes of the movie and really say, 'Oh, well, what would happen if one of these things was loose in my neighborhood?' It's definitely fun to watch the guys jump in the robots and fight them, but I think you also want that drastic quality of what happens when the dinosaurs are loose on the streets."

One of the sets that we saw during the day was a room where Newt tries to hide underground, only to have a kaiju dig its way down to him, flooding this room with dirt and water on top of the 300 extras already surrounding Newt. We asked him how much acting you have to do when you've got that many real variables already in the mix. "I used to live pretty damn close to Chinatown in New York, so that was a little bit of preparation, and getting off the Grand Street Station at 6:30, I've been there. It's great because it does kind of help you with the acting a little bit in that it's not just standing in front of the giant green screen and imagining the people. You really do get thrust into the environment. He's good at building these environments, which has been great. I think it's something that will be cinematic at the end of the day, that is not just all CGI, that it will be the combination of CGI with practical sets and little models that he's built and puppets… all those fun Guillermo things."

Del Toro tends to cut while he's shooting, so we asked Day how much he'd already seen and if it lived up to what he imagined when he read the script for the first time. "What I keep forgetting is that there's this other movie filled with giant robots and giant monsters that's going to be added to this movie. So it's exciting when you see the good scenes and then you hope that, you know, mixed together with the other stuff, that it's going to be great."


The size of the film and the non-stop nature of some of the action is ambitious, no matter what the budget is, and Day seemed confident that it was a potent combination. "I think that's one of the most exciting things about the movie. I hope it all comes together because it has the potential of being one of those big scary action-packed films, like 'Temple of Doom' or 'Jaws' where there's a threat of real danger but it's also a big fun ride. I just hope that it all comes together in the right way."

So much of the movie deals with creating this alternate world with these big ideas like The Bridge and neural networking and alternate dimensions, and when I read an early draft of the script, there was an actual glossary included up front. We asked if it took a while for Day to get comfortable with the terminology. "That's an interesting question and it's true. There was a bit of a learning curve with language because often times, especially with 'It's Always Sunny in Philadelphia' or 'Horrible Bosses,' I can learn the lines very quickly and then we can throw it away and riff. But when you're talking about the intricate inner workings of the kaiju, you really can't. It just doesn’t come to you spontaneously. I definitely had to study the lines more intensely than I ever have before."

Everyone who has worked with Guillermo Del Toro ends up doing a Guillermo impression. It's inevitable. There's this infectious quality to Guillermo that is irresistible, and we asked Charlie if he did one. "I can't. It ends up coming out as this cartoon Mexican voice. It ends up like those old racist cartoons, the ones with the mouse that, like, ran…"

Someone offered up, "Speedy Gonzales?"

"Yeah," Charlie replied. "So I don't think I can do that."

We told him that Perlman had claimed that he does the very best Guillermo impression, and Charlie started laughing. "Yeah, well, Perlman's biggest fan is Perlman." Still laughing, he corrected himself. "I really like the guy, for the record. I'm going to be in so much trouble."

With that, they called Charlie back to set, and threw himself back into his struggle to evade the giant invisible monster, and no matter how tired he looked, that smile never seemed to completely go away.

"Pacific Rim" is in theaters July 12.