Much to his surprise, Al Pacino learned that once upon a time he met the legendary music producer Phil Spector, whom he now plays in a new HBO film.

He had no memory of it, "but somebody showed me a picture of me and him on the Internet," Pacino laughs. "It was at some event or party, and we're both looking into the camera, two guys who do not want to be photographed. Since he had mostly worked behind the scenes, I didn't know who he was, and he looked like he didn't know who I was."

That was then, whenever that was. Now, spurring after-the-fact speculation, this forgotten encounter serves Pacino as a fitting first step into the character he captures for "Phil Spector." (It premieres Sunday at 9 p.m.)

Written and directed by David Mamet, this penetrating film explores the preparation for Spector's murder defense: As the story begins in 2007, he stands accused of having forced a pistol into the mouth of a woman - his by-chance date for the night - and pulling the trigger.

The difficulties of the case seem beyond the wherewithal of Spector's original attorney (Jeffrey Tambor), who has brought in hotshot lawyer Linda Kenney Baden (Helen Mirren). She takes over as lead attorney and, as the film unfolds, joins Spector in a verbal pas de deux that teems with Mamet's shrewd dialogue:

"Why do you have so many guns?" she inquires on her first visit to his home near Los Angeles, where the shooting took place.

"I might need one," he replies.

"Why would you need more than one?"

"How many shoes do you have?" he poses. "How many feet?"

Spector is gnomish, unstable and grandiose: "Extraordinary accomplishments," he says, meaning his own, "transform the grateful into an audience - and the envious into a mob."

So he's a problem for his lawyers. He says he didn't kill the girl, but who's going to believe him?

His lawyers know that, in the mind of the modern-day public, he is not the music wizard who created the girl-group sound in the early 1960s, co-wrote "You've Lost That Lovin' Feelin'" for the Righteous Brothers, and produced records for the Ramones and the Beatles. Instead, by 2007, he's generally regarded as a creepy, homicidal has-been who hides out with his dozens of guns and his dozens of outlandish wigs.

As Mamet portrays it, that's the lofty challenge facing Spector's legal team.

But "Phil Spector" makes no claim to uncovering the true facts of the crime, or of Spector's guilt or innocence. Quite the opposite. The film opens with a flat-out disclaimer that it's NOT "based on a true story," that it's a work of fiction "inspired by actual persons in a trial," but unconcerned with depicting those actual persons or the case.

"That's why it's set in trial preparation," says Pacino. "We weren't there. No one knows what happened in the trial preparation." Or no one, anyway, who will talk about it. Which makes the inside story ripe for invention by Mamet, the Pulitzer Prize-winning writer whose many works include the play "Speed-the-Plow" and screenplays for "The Verdict," "The Untouchables" and "Wag the Dog."

"Here's the thing," explains Mamet by phone from Los Angeles. "I'm a gag writer. That's how I make my living. I don't want the audience saying, 'That's the most accurate thing I ever saw in my life.' That's what we have dictionaries for."

But there are a few inarguable truths about Spector. One is that in 2009, he was convicted of second-degree murder and, at age 68, sentenced to 19 years to life.

Another is that, for record buyers of a certain age, his music reigned as the soundtrack for youthful seduction. As Spector tells his lawyer brazenly, "Linda, the first time you got felt up, guess what? You were listening to one of my songs."

Indeed, the 65-year-old Mamet was among those who grew up with his songs, albeit without giving much thought to some Phil Spector guy.

But in recent years, he became fascinated with Spector and the 2003 shooting and the murder case, especially the many loose ends (like: did Spector do it?) that may never be resolved.

For Mamet, this was perfect. For these storytelling purposes, the truth is what he makes of it.

"What great writers do is to take an idea from a character who really exists, and revise it, and make a character out of it who can express what they're going for," Pacino says. "And the character becomes an extension of him."

And an extension of the actor in the role. So Pacino (whose rich history with Mamet includes starring onstage in his "American Buffalo" decades ago and in a Broadway revival of "Glengarry Glen Ross" last year, besides appearing in its celebrated 1992 film version) signed to build on Mamet's imagining of Spector.

Unlike a film such as Pacino's 1973 crime drama "Serpico," where the real-life undercover cop Frank Serpico was readily available for Pacino to spend time with, he never tried to visit Spector in prison. It would have served no purpose, he reasoned. The incarcerated Spector was a different man from the Spector who had once been facing trial.

Instead, Pacino read all about Spector and screened hours of archival footage.

"I try to get that thing that's inside of someone and connect it to myself," he explains. "I like to allow it to get into my unconscious. How, is a mystery to me. But as Jackson Pollock said, 'When I see what a painting is that I'm painting, I move on or I destroy it, because I'm no longer in my own unconsciousness.' I approach characters that way."

Pacino heaps praise on Mamet, who, in directing his own script, might have been too protective of the words he wrote, but wasn't.

"He was surprisingly flexible," says Pacino. "There's probably things in his writing that he's partial to: 'Play 'til the lemon chicken comes'" - a riotous line Spector fires out in a scene in the recording studio - "you've GOT to say that! But all in all, I found him to be flexible, and that relieved me."

Meanwhile, the jagged on-screen chemistry Pacino shares with Mirren found its roots in their warm working partnership.

"I love her!" he says. "I just can't tell you the advantage of having someone you can talk with every day, just to get her read on things. Sometimes I thought, 'I'm talking too much to her. She's gonna start getting bored.' But she was a real mensch!

"And that's my memory of the film," he sums up with a laugh: "Her. And me struggling to get through my scenes with those wigs!"

Pacino is now mulling future projects, including a long-rumored Napoleon biopic.

"But I don't think I'm ready to go out there and just do a job," he adds, growing serious again. "Sometimes I've made movies because I was having a tough time in life: You know, go make a movie, get away from THIS world. But I don't think I have that kind of energy to do that anymore. I only get turned on by something with a challenge, where there's an opportunity to do something with it.

"It's comforting," he declares, "to feel like that possibility exists."