But the actor known for his collaborations with writer-director Paul Thomas Anderson in "Boogie Nights" and "Magnolia" said he relates to the feelings of "Mr. Cellophane," the invisible alter-ego of Amos Hart, the schlubby pushover he plays in the movie "Chicago."

"I've been busy doing stuff like this lately," Reilly said of musicals at a recent press event for the Miramax movie version of the John Kander-Fred Ebb-Bob Fosse musical — which debuts in select markets Dec. 27 before opening wide in January. "They called me up and asked me if I wanted to do it, provided I could sing, which I had to prove to them through videotape. I was thrilled because I'm a big fan of musicals and I grew up doing musicals. I did nothing but musicals, actually, from the time I was about eight years old to the time I was 18. Not by choice, really, because that's all there was to do where I grew up, was musicals or... crime. So I chose musicals and that's where I learned to be an actor really. I was thrilled when this came up."

The veteran of such musicals as Brigadoon, Pajama Game, Peter Pan and Jesus Christ Superstar revealed "I had already been investigating musicals in other ways. Paul Thomas Anderson and I wanted to do musicals. We were talking about doing musicals at one point and I got involved in this production of Marty, which is a musical based on the movie which I ended up doing in Boston after I filmed this movie. So I'd already been poking around in the musical world again."

Reilly joked about the chance of an upcoming Anderson movie musical: "He's still saying [in his P.T. Anderson voice] 'Wait til I do my musical.' But Paul's not the most collaborative writer, he's a very collaborative director, but when he's writing, he's just like 'What is he building in there?'; hearing clanking of pots and pans and he comes out with beakers bubbling and he's got some new thing. So, we'll see."

In the movie "Chicago," the musical numbers are an offshoot of showgirl wannabe Roxie Hart's imagination, in the film's reality-based setting of 1920s Chicago. As such, the cast needed to call upon singing and dancing skills while still asserting their acting chops. "I think [director] Rob [Marshall] really wanted me in this movie," he said. "The feeling I got was he was hoping I could sing and he was hoping I could dance, but what he really wanted me for was the acting. That was his big thing; he knew he could get the numbers right because that's his background."

Director-choreographer Marshall is a six-time Tony Award nominee (Damn Yankees, Little Me, She Loves Me, Kiss of the Spider Woman and Cabaret).

Reilly added, "[Marshall] wanted to really capture Chicago as it was in the 1920s, to really make the scenes very realistic, so that when it jumps to the fantasy number it was a real switch as opposed to [integrating] the whole thing into [a] more traditional musical style."

The only native Chicagoan in the cast, Reilly sulked about the movie's mostly Toronto-based filming locations: "We recorded the vocals in Toronto, they recorded the orchestra in London, then [we] filmed almost all the movie in Toronto too, except for some exterior shots which broke my heart. I would have just died and gone to heaven to shoot this movie in Chicago."

Amos Hart's big song, "Mr. Cellophane," is envisioned in the movie as a tramp-clown number with Reilly donning makeup and tattered clothing and performing a somewhat soft-shoe dance. "The inspiration for the song 'Mr. Cellophane' — I heard from Kander and Ebb — was this guy, Bert Williams, who was a famous black vaudevillian performer who was the biggest star of his day," Reilly said. "He had this great song called 'Nobody' and it was similar to 'Mr. Cellophane' in that the sentiments were kinda the same. Unfortunately, you can't really see Williams perform anymore and his stuff is kinda hard to get ahold of, but I did a little research on him."

Reilly called upon other performers as well. "I felt like I understood the pathos or the psyche or the archetype or whatever it was, from all the people that I'm fans of: Stan Laurel, Bert Lahr, Red Skelton, Dick Van Dyke, Emmett Kelly, Charlie Chaplin, Buster Keaton," he explained. "There's this long lineage of tramp clowns and I actually did a lot of clowning as a kid. It was another thing I did for cash, I had a tramp character."

With all those influences and research in mind, Reilly revealed his Mr. Cellophane came partly from his own buffoonery. "So I was thinking about a lot of those guys when Rob and I got together to come up with the number. When it came to choreograph, he'd say 'Well, what do you wanna to do, John?' We'd just kinda move around, I'd start moving around, even like joking. In the number, the part where I kinda sweep my feet side to side, I was just horsing around like that and he said, 'That, you're gonna do that in the number.' And I was like, 'Wow... Alright. I was kidding. Well, it's easy for me to do, so great.' I think that's one of Rob's secrets, in terms of working with people who aren't dancers first, is finding natural movements that just come out of them and incorporating that kind of organic stuff. He said something the other night in a Q&A, 'I'm always watching out of the corner of my eye for the person whose doing the steps wrong because they might know something that I don't know. I like to give people something to do and I watch them and if it's working against someone's instincts sometimes watching what they do can give me a great new idea.'"

The actor had the luxury of not seeing any other actor perform the role, he confessed, "For all I know this is the way it was supposed to be. I feel lucky that I haven't seen anybody else do it because I felt free to just bring whatever I was going to bring to it."

Reilly also had that luxury when creating the musical version of Marty which is expected to reach Broadway in the near future. "It was a big success in Boston, the audiences went nuts for it," he said.

Would Reilly star in a Broadway run of the tuner? "I would, it would be a big undertaking, I have to say. I'm not saying I'm ready to do it because it would be a big decision for a lot of different reasons, but they got people who want to put up the money, I guess and they're just looking for a time and a place to do it. We'll see."

Aspirations for any other musicals? "I think the one that got away was Guys and Dolls," Reilly said. "That is one that I would love to do someday, it's been done recently, so it'll probably be a while. I'd love to do Sweeney Todd one day too. I try to choose things that I really believe in, that there are reasons to do, besides getting paid or becoming well-known and I think that's paid off in a long-term business strategy so far. It's a little tough day-to-day, I still have to fight really hard to get work, I have to audition and prove myself over and over again. I've done about 30 movies now, I feel like I've kinda proved myself already, but if you're not a household name and not on the public consciousness, people treat you like 'What are you going to do for me?' It's something that I was thinking about a little bit when I was doing 'Mr. Cellophane,' I have certain personal associations with the sentiments in the song. Maybe this will be the one where people start to realize, 'Alright, let's just give him the job.'"