Judging by the rapturous response he's received at Sundance this past week, formerly drug-bedeviled star Robert Downey Jr. is back with a vengeance.

It was standing room only at Friday's world premiere of The Singing Detective — the Oscar-nominated actor's first leading role since completing a very public process of rehabilitation and Downey's performance as a bitter, misogynistic novelist suffering from a debilitating skin disease is terrific.

But the 36-year-old actor is reluctant to make a big deal about his reappearance on the big screen after a three-year hiatus.

"Don't call it a comeback!" he bellows half-jokingly, before relenting and sharing a favorite quote from an L.A.-based inspirational speaker.

"He says, 'The bigger the setback, the bigger the comeback' and I love that," Downey says.

Though his thick black hair is graying at the temples, Downey's antic class-clown personality remains unchecked throughout his sit-down with The Post, he dishes on-set anecdotes and fires off a relentless barrage of quips and impersonations.

"People ask why Robert has been given so many second chances and I think it is because, not only is he an incredible talent, but he's also such a kind and gentle person," says "Detective" director Keith Gordon.

"He's like the sweetest puppy dog - it's almost impossible not to wish him well."

Downey lost his last job playing Calista Flockhart's love interest in Ally McBeal after his arrest on cocaine-related charges in Culver City, Calif., in April 2001, five months after a similar incident in Palm Springs.

Mel Gibson a friend of Downey's since they starred together in 1990's Air America promised his pal he would find a role for him once he got clean.

When Gibson's Icon Productions acquired the rights to Detective 18 months ago, he made good on his vow.

"It's an amazing role for Downey it brings out a lot of his best qualities," Gordon says.

"Detective," which Gordon calls "a surrealist musical comedy drama noir thriller," is a remake of Dennis Potter's classic BBC TV series.

Downey plays pulp-fiction author Dan Dark, who suffers from writer's block, a marriage breakdown and a crippling form of the skin disease psoriasis, so extreme his hands were fused into fists.

Bedridden and feverish from pain and massive doses of steroids, he mentally rewrites himself into one of his characters, a hard-boiled private eye who moonlights as a singer.

Downey spent from four to six hours a day having prosthetic makeup applied, and was sometimes required to lie in bed with his hands curled for up to 15 hours at a stretch.

"I didn't envy him," Gordon says. "It was like, 'Welcome back to the world of film-making. You took three years off to get clean and now look at this part.' "

The shoot was "surprisingly easy," says Downey that is, except for the makeup.

"By the time we got the preparation out of the way," he recalls, "I was like, 'Let's get this on film because I feel like I have shards of glass glued to my eyelids, you masochistic f- - - - s.' "

Downey jokes about drawing on his real-life experiences for the hallucinating scenes.

"Well, you know I'm definitely a big tripper, I love the 'shrooms and all that stuff," he says.

"But when you're making a movie you're not drawing on much except, How can I be as good as I can be in this scene?"

And so if people are drawn to the film out of curiosity at seeing Downey in his comeback role, that's fine with the director.

"Luckily, I think Robert has done an amazing job," he says. "Mel even said to me it was a shame we couldn't get the film out in time for Oscars."