Is his pen mightier than the wand?

David Thewlis doesn’t look anything like his most famous cinematic character, Remus Lupin, the lycanthropic Hogwarts wizardry professor from the “Harry Potter” movies. In fact, shambling down from the L.A. home he shares with his companion, actress Anna Friel -- now a star in her own right with her critically acclaimed new TV series “Pushing Daisies” -- and their 2-year-old daughter, Gracie, Thewlis bore a much closer resemblance to his formerly most famous film persona, the downtrodden nihilist Johnny from Mike Leigh’s 1993 low-life picaresque, “Naked.”

Tall, unshaven, in ravaged jeans and a shirt based on a castoff Army-surplus design, Thewlis cut the figure of an artist detached from mundane concerns. That he has now become a first-time novelist, with the U.S. publication of “The Late Hector Kipling,” only completed the image.

“I love L.A.,” he said, from behind gold-rimmed sunglasses as he fired up a Marlboro Light, the first of several indications that Thewlis hasn’t completely abandoned the vices and demons that he channeled so productively on-screen in “Naked.”

As for the novel, which he started working on seven years ago: “I knew it would be a black comedy. And I also knew it would be about a descent into madness.”

Thewlis was speaking from the sun-blasted sidewalk in front of the Bourgeois Pig, a noir-ish writers hangout across the street from the Scientology Celebrity Centre on Franklin in Hollywood. Thewlis makes no effort to mask or modulate his northern English accent, a vestige from his youth in Blackpool, a working-class resort town on the Irish Sea where he was raised by shopkeeper parents. As with much else about the 44-year-old, it contributes to the impression that he’s devoted to revealing the truth about human experiences. He’s in the unique position of being an actor whose fans believe that he doesn’t really act so much as decant his tormented personal reality into his characters. “Naked” -- with its improvised script, scathing indictment of Thatcherite England and unflinching view of the dark side of human interactions -- defined this aspect of Thewlis’ reputation.

But excessive truthfulness threatened to stall -- or at least complicate -- Thewlis’ writing. In at least one early story, based on the filming of 1996’s “The Island of Dr. Moreau,” he simply fictionalized the surreal happenings on director John Frankenheimer’s set. He was surprised that his editors thought he had actually made it up.

Troubled artist

“The Late Hector Kipling” is a more thoroughgoing work of invention. The novel recounts the sordid tale of the title character, a famous but discontented middle-aged London painter, as a rift with a longtime and suddenly-more-famous artist friend leads to what Thewlis termed a “will to self-destruction.” The time frame is the recent past, and the milieu is populated by various actual members of the “Young British Artist,” or YBA, scene. The YBAs, whose ranks include Tracey Emin and Damien Hirst, gained notoriety in the 1990s for shaking up the English art world through disturbing imagery and installations as well as by adopting an iconoclastic public stance.

Thewlis, an art collector and occasional sculptor, found this demimonde, with its outsized egos and masculine jousting, to be an ideal setting. “I wanted to write about issues of rivalry and creative theft,” he said about the relationship between Hector and Lenny Snook, friends since their teenage years who have risen to the top of the art world. Lenny does conceptual art. Hector paints huge heads that sell for five figures. Lenny, however, has recently been short-listed for the prestigious Turner Prize (a real award, handled by the Tate Gallery, that defined the careers of many YBAs). This flips a switch in Hector’s envious psyche, setting in motion an orgy of betrayal, sadomasochistic sex, boozing, chain-smoking, slaughter and eventual incarceration (there’s also a murdered dog, a hideous sofa, a brain tumor and a madman whose father molested a horse). By the novel’s end, the bodies literally pile up on stage, leading some reviewers of the British edition, published by Picador earlier this year, to accuse Thewlis of overdoing it.

“Shakespeare did the same thing,” he said, countering the criticism with the biggest stick he could find. By and large, however, the reviews have been positive.

Thewlis writes in an energized, propulsive style, peppered with slang, profanity and rude, inventive metaphors (the carpet of a dive bar “has cancer”; a river “stinks of pigeon”). His prose demands attention. It’s that style, coupled with the career-rivalry plot line, that made comparisons perhaps inevitable to Martin Amis’ “The Information,” a similarly revved-up novel that chronicled a snit between a pair of English writers.

Thewlis shrugged this off. “I never read it.”

Instead, he attributed his sophisticated, high-wire-act style to being a voracious reader. “I know more about books than films,” he said, citing Samuel Beckett’s novels as influences, but also paying homage to American writers, such as Philip Roth, Raymond Carver and Kurt Vonnegut.

In addition to the artist-versus-artist theme, Thewlis has also used the novel to examine the fragile nature of love. “When love starts to slip, it’s a terrible feeling,” he said, echoing Hector’s inability to salvage his relationship with a beautiful Greek musician. “Hector doesn’t understand love,” he insisted. The writer, however, does.

“It’s ironic that I found love on the night I finished the manuscript,” he said, recalling his first meeting with Friel. Although he and Friel, who plays Charlotte “Chuck” Charles on the whimsical ABC series, are the happy parents of a young child, Thewlis maintained that he hasn’t completely shaken off the memories of his previous failed relationships. “I’m not as self-loathing as I used to be, but I’m still preoccupied with the themes.”

Taking up the pen again

There’s another novel in the works. “I’m working on something more ambitious this time,” he said. “It’s set partly in L.A. and tells a generational story. I definitely want to write more. It’s such a pleasure when it goes well.”

“David is an incredibly thoughtful novelist,” said Sarah Hochman, his Simon & Schuster editor. “Underneath all the darkness in ‘The Late Hector Kipling,’ there’s a kind of depth. He’s a little older than the typical debut novelist. His life experiences clearly inform his work. He’s particularly talented at telling a wild story in a way that ends satisfyingly.”

And what was it like to collaborate with “Naked’s” Johnny and “Harry Potter’s” Remus Lupin? “I was immediately intrigued,” she said. “I couldn’t wait to see what this person had done in a new medium.”

Thewlis, whose filmography is closing in on 40 movies, is content with his growing reputation as a Renaissance man -- although, of course, the acting thing isn’t going away. He has at least two more Potter films to look forward to (he said that he immensely enjoyed the process of making the first two, a contrast with some of the projects he’s worked on). Still, his latest career direction has fulfilled him in ways cinema couldn’t.

“I always had the ambition to write,” he said. “When I was commissioned to do ‘Hector Kipling,’ it was the greatest day of my life.”