From comedy to country, in the key of John C Reilly

The Hollywood star is one of the most dependable film actors around, but after performing songs on screen he decided to keep the musical show on the road

'I'm in support of people expressing themselves artistically, regardless of their background, of what their day job is. I think singers should be able to sculpt, actors should be able to sing, and politicians should to be able to knit if they want to. What's wrong with that?"

Nothing at all, John Christopher Reilly, nothing at all. It's just that, occasionally, the part-timers aren't very good at their hobbies. Reilly, however, happens to be quite the dab hand at fusing a certain level of the here and now with Americana, bluegrass, old-time, countrypolitan, bare-boned folk, sea shanties and the Everly Brothers. Is it because he's part-Irish?

"Well, that's definitely an influence," says the Chicago-born actor, who arrives in Ireland this weekend to play a gig in Dublin with his self-styled (and C-minus) John Reilly & Friends unit.

"I grew up listening to a lot of traditional Irish music, for one thing; that's what my dad was really into, and then my mom, whose heritage is Lithuanian, was into listening to older music - the kind you could hear on those old piano rolls. As I got older I taught myself to play the guitar, became interested in bluegrass and American roots music. And, of course, as many people know, the deeper you go into that, the sooner you end up back in Ireland. That's full circle and quite unavoidable, but that's just the way it is."

Music is something Reilly has always loved; he grew up listening to musicals, performing in them and going to see them. Then, as his film career took off, he was increasingly asked to engage with music in a public way. As naive Amos, husband to floozie Roxie in Rob Marshall's 2002 movie version of the musical Chicago, Reilly performed one song (Mr Cellophane), but it wasn't until Robert Altman's 2006 A Prairie Home Companion, the director's final work, that Reilly sang to a great extent on camera (as Lefty, the singing cowboy). Then came Jake Kasdan and Judd Apatow's 2007 music biopic parody Walk Hard: the Dewey Cox Story, which featured Reilly singing all the soundtrack songs. He also toured as Cox in the lead-up to the film's release in the US. "One day I woke up," Reilly says, "and thought it seemed a shame to stop playing music just because any given movie was over."

Storytelling

It's important to Reilly that people understand he is doing music not so much as a stopgap between film roles (the latest of which is the hugely successful Disney animation feature Wreck-It Ralph) but as a creative necessity.

"It's all storytelling, isn't it?" he says. "All my favourite songs have beginnings, middles and conclusions, and that's true with the movies and theatre work I do. If you're doing a play or a movie with a very dramatic scene, in order to get to the heart of the audience you first gotta go through their brain. If there's something in common with what you do and what the audience feel, then you've hit it, and the result is moving in an emotional way.

"With music, it's more of a direct line into people's hearts; I've heard a lot of songs over the years that I don't understand what a lot of the words mean, but I'll hear the melody and it gets me each and every time. It stays with me."

A city with dozens of theatres, Chicago is rightly famous for nurturing actors, but there is a notable commercial difference between "working" and "successful". After high school and a sequence of amateur plays and musicals, Reilly studied at the city's Goodman School of Drama and the renowned Steppenwolf Theatre Company. He might be far better known these days as one of the most dependable and distinctive-looking film actors around (he featured in The Good Girl, Gangs of New York, The Hours, Magnolia, Step Brothers, Cyrus and We Need to Talk About Kevin) but a deep love remains for the stomach-churning charge of theatre.

"One of the reasons I became an actor," he says, "is because I have trouble defining what the intrinsic characteristics of my personality are. My approach with things - acting, music - is simplicity and honesty. If people find me simple and honest, then that will make me happy. That's the approach I take - I certainly don't like to over-complicate things."

Whatever work he undertakes, he says, is down to his south Chicago working-class background, and the fundamental need to be employed. Stage, song, drama, comedy, musical, lead, support or cameo, traditional media or online (he appears regularly with friend Will Ferrell in sketches for comedy website Funny or Die), it matters not once the cheque is in the post.

Fickle business

"Coming from the background I'm from it makes you hyper-aware of how fickle the business of Hollywood and the entertainment industry in general can be," says Reilly. "The closest anyone in my family ever got to the stage before me was one of my grandfathers, who was a bouncer at a burlesque show. He used to stand in front of the stage at a Chicago theatre, making sure the guys in the audience wouldn't storm it."

Reilly is a self-made son of an Irish man living the dream in Hollywood, then? "Too true," he replies with a chuckle. "It's an extremely unlikely turn of events that I'm here, doing what I do. It's never lost on me how lucky I am."

Reilly in the groove: In film and song

Chicago (2002):Directed by Rob Marshall, the film version of John Kander and Fred Ebb's record-breaking musical features Renée Zellweger, Catherine Zeta-Jones and Richard Gere. Reilly was nominated for an Oscar and Golden Globe in the best supporting actor category.

A Prairie Home Companion (2006):Director Robert Altman's final movie is about writer Garrison Keillor's public radio show of the same title. Reilly plays Lefty, the singing cowboy. Yee-haw, etc.

Walk Hard: the Dewey Cox Story (2007):Music biopic comedy written and produced by Judd Apatow and Jake Kasdan (who directs). The plot apes the storyline of 2005's Walk the Line, and sees Reilly lampoon numerous rock'n'roll and pop stars through the decades.

Funny or Diecomedy website featuring original material from the likes of Reilly, Will Ferrell, Judd Apatow, Anne Hathaway and Ben Stiller. In one clip, Reilly and Ferrell re-enact Bing Crosby and David Bowie's Little Drummer Boy video. funnyordie.com

John Reilly & Friends perform at the Dublin Unitarian Church, St Stephen's Green, on Saturday. Wreck-It Ralph is released tomorrow