There’s a somewhat scurrilous character called Scagnetti loose in Tom Sizemore’s West Hollywood digs, but it’s not the identically named, relentlessly self-promoting detective Sizemore portrayed in Natural Born Killers. It’s an 11-week-old golden retriever that’s spent the better part of the day besmirching his master’s territory. “He’s pretty housebroken, by and large,” says the actor with a sigh. “But every once in a while he just....lets it go.”

Fortunately, Sizemore is having too prosperous a career to let an incontinent canine ruin his day. Currently the big screen’s highest-profile supporting player, he has - in the last 12 months - completed three major films alongside Oscar-calibre co-stars and landed the male lead in one of next summer’s big releases. In the first of these projects, last fall’s Devil in a Blue Dress, he was the enigmatic man who lured detective Denzel Washington into a dangerous game. Strange Days found him hanging out with hardware hustler ralph Fiennes, and this winter’s Heat had him staging heists with commando-style bank robbers Robert De Niro and Val Kilmer.

Sizemore’s latest, The Relic, a pricey Peter Hyams-directed horror opus, will move him to the top rung of the casting ladder - and away from the largely sociopathic souls for which he’s become known. Pitching the film as “Alien meets Die Hard,” Sizemore stars as police lieutenant Vincent D’Agosta, a recently divorced homicide detective who teams up with an evolutionary biologist (Penelope Anne Miller) and follows a trail of gruesome slayings to the basement of Chicago’s Museum of Natural History - where he begins to suspect that the killer isn’t merely inhumane, but inhuman. “I don’t want to give too much away, but it’s very frightening,” he says, nevertheless divulging that the story pivots on a DNA experiment gone awry a la Jurassic Park and Species. “This is a better story than Species, though, and I think a much better movie. It’s very literate. The characters are all really three-dimensional.”

As scary as the movie’s plot may be, the 33-year-old actor - despite having had a hand in rewriting and customizing his part - has other, real-life fears. “Sure, I’m psyched, excited, all those things,” Sizemore says. “But doing this movie wasn’t an easy decision. I’d been waiting a long time to step up and play a lead, and I just wasn’t sure that this was the one I wanted to do it in. Then again, I guess I’d have been nervous no matter when the opportunity came.”

Sizemore, who habitually turns down better-paying gigs to pursue pedigreed projects with stellar filmmakers (he’s already worked with some of the best, including Oliver Stone, Carl Franklin and Kathryn Bigelow), sought the advice of his good friend Robert De Niro while weighing The Relic. “I was in New York when I got the offer, and I got the offer, and I discussed it with Bob over lunch. He asked me, “Why are you worried?” And I said “Well, I’ve never had to carry a film.” He said, “Just take it one scene at a time. Stay healthy, get your rest - and it will happen.” So I’ve been trying to practice that edict. I’m just taking it one scene at a time, one day at a time, and trying not to look ahead. Because it looks like a mountain sometimes!”

Highly polished in performance, if slightly rough-hewn in appearance, Sizemore doesn’t hesitate to say that he works harder than most. Doing homework, he feels, is critical to cracking a role. For The Relic, he met with curators and scientists to learn more about the research that goes on behind public museum exhibits. And for the part of lethal larcenist Michael Cheritto in Heat, he spent time at Folsom Prison, a maximum-security facility near Sacramento, California, to gain insights into criminal psychology and felonious “crews” like the fictional one his character is part of in the film.

Sizemore’s trip to Folsom was particularily eye-opening. “I said to this one convict, “Your rap sheet says you killed five people. You wanna talk to me about that?” He goes, “I only killed two people.” I said, “Well, it says here five.” And he goes, “Oh, those others were cops.” He said it in a really non-dramatic, flat-toned way. So I pressed him and said, “Well, I don’t know how you view them from your world, but I think most people would think that police officers are people - with wives and children and loved ones.” He just looked at me blankly and went “Uh-huh.”

“It was during the lengthy prep time and shooting of Heat that Sizemore cemented his friendship with De Niro, who helped convince director Michael Mann to cast him in the film. The two first met briefly when Sizemore did a walk-on in the 1991 drama Guilty by Suspicion, then ran into each other at an LA restaurant a couple of years later. The actor fondly recalls the moment when the two-time Academy Award winner invited him over to his table and complimenting him on his turn as Jack Scagnetti in Natural Born Killers.

“I ended up sticking around there that night,” says Sizemore. “Bob was with Sean Penn, who I’m friends with, and we just kind of all hung out and talked movie talk, girl talk, whatever. Truthfully, I was a bit nervous about being in such close proximity to him. But I could tell he liked me. Since then, it has moved into real friendship. He’s kind of adopted me in a sense. He’s always calling me and making sure that I’m getting my rest.” Sizemore pauses momentarily, no doubt pondering how fate brought him so close to the actor whose work helped inspire him early on. “Of course, I always idolized Robert De Niro. I was a huge fan of his. I saw The Deer Hunter and Taxi Driver many times as a young actor wannabe.”

Growing up in Detroit, Tom Sizemore certainly had his share of dreams and disappointments. Describing himslef as “a happy athletic kid,” he was the first of three boys born to poor parents who were barely out of their teens - themselves the children of migrant farmers who worked in the factories during World War II. A complicated, taciturn, but exceedingly intelligent man, Sizemore’s father won an academic scholarship to Harvard and taught philosophy at two other institutions before switching careers and attending law school. After graduating, he moved the family out of Detroit’s harsh environs to the suburbs. “My parent’s marital problems started almost immediately after we relocated,” says Sizemore. “They separated about a year later, when i was 15, and we moved back to Detroit with Mom. I rebelled against my father - even though, in many ways, he was my closest friend at the time.”

Though still an athlete who excelled in football and basketball, Sizemore began losing focus while attending an all-boys Catholic high school filled with “wealthy kids from Grosse Pointe. It was a fast crowd; there was a lot of permissiveness. I was kind of careening through life. My buddies were all either jocks or dopers - a very macho group. Two six packs a night, six or seven joints. We started getting into coke. It was really fucked up.”

In the midst of this tumultuous period, Sizemore met his girlfriend, an aspiring actress. “She coerced me into trying out for a play, which I did against my better judgement because I thought I’d be teased to death. It was a small part in The Music Man. I know it’s a cliche, but it was like a light went on.” At 17, after performing in a summer-stock run of Bye Bye Birdie, he decided to become an actor. “It was not a popular decision with my parents. But I was still kind of wild, and they were happy I found something I wanted to do.”

Following graduation, Sizemore enrolled at Wayne State University as a theatre major and became the star of the department in dramas like The Glass Menagerie, Waiting For Godot and A Christmas Carol. “I don’t mean to brag,” he says, “but I was far and away the best actor there. And I felt myself becming better.” Broadway beckoned, but Sizemore, then only 20, was apprehensive about making tracks to Manhattan. After picking up his undergraduate degree, he opted to go for his master’s and auditioned for both the Yale and Temple University drama schools - signatories of the prestigious League of Professional Actors Training Program, which also seeded the likes of William Hurt, Kevin Kline, Meryl Streep and Robin Williams. Landing at Temple, Sizemore again impressed his teachers and his audiences and finally built the confidence he needed to survive in New York.

After moving to Greenwich Village and dabbling in commercials under the guidance of a “teeny-weeny” manager, Sizemore - who laboured as atruck-loader for UPS and Coca-Cola, among other decidedly unglamorous jobs, to make ends meet - joined the Ensemble Studio Theatre and did off-Off Broadway plays while seeking an agent. An introduction to two casting directors led to his signing for a movie called Dogfight as Matt Dillon’s sidekick, but the project fell through (the film was later restarted and finished with River Phoenix in the lead and a new supporting cast). Yet the connections he’d made in the process got him into one of New York’s leading agencies, which sent Sizemore to a successful audition for Oliver Stone’s 1989 Vietnam drama Born on the Fourth of July. His small role in that film gave rise to parts in Blue Steel and Lock Up - the latter earning him a ticket to Los Angeles and entry into the powerful Creative Artists Agency. “That’s when I began my career,” he says, noting subsequent films like True Romance, Striking Distance, Hearts and Souls and Passenger 57. (Sizemore’s TV work includes a stint on China Beach and the Hallmark Hall of Fame movie An American Story.)

Around that time, Sizemore began to be seduced by what he now calls “that Hollywood fuckin’ jive-ass scene. I was burning it at both ends, staying out late and going to the Monkey Bar, the Whiskey. It was self-destructive behaviour.” The actor credits his pal De Niro and his own girlfriend, Maeve Quinlan - a former professional tennis player and a regular on The Bold and the Beautiful - with helping him clean up his act.

“Maeve was instrumental in giving me a fulfilling life at home and telling me that I was being used by people,” he says. “There are a lot of peripheral people out there - the Hollywood Fringe - who get high on a young actor being around: it gives the evening energy. And I just got sick of it. I got sick of all the girls, the drugs, the booze....the bull. I have big plans, and I came to realise that they weren’t going to get done that way.”

Other than an expressed desire to work with Stanley Kubrick, Jack Nicholson, Jodie Foster and Bernardo Bertolucci, Sizemore remains vague on his professional plans - although his newfound domesticity (he’s also godfather to buddy Michael Madsen’s youngest child, Hudson), appears to be pushing him towards “quieter movies with more relationship and love issues.” On the personal front, he happily reveals, looms his imminent marriage proposal to Quinlan.

Maeve, of course, would have to accept Tom’s bad habits along with his hand in marriage. Aside from smoking cigars, the actor admits to a near-fetish for fresh briefs. “I change my underwear a lot,” he says. “When I was poor in New York, I only had, like, three pairs. I swore that if I ever got any money, I’d never, ever, ever wear dirty underwear. So I’m perpetually changing it.”

Er, how often is “perpetually”?

“At least twice, sometimes three times daily,” Sizemore chuckles - even as it dawns on him that airing his dirty laundry might make headlines. “That’s enough,” he says, checking his watch. “I’ve gotta go!”