By Lisa Richwine

PASADENA, California (Reuters) - Playwright David Mamet had little interest in legendary music producer and convicted murderer Phil Spector, dismissing him as a "freak" - until he watched a documentary that shed light on a complicated personality.

Now, the "Glengarry Glen Ross" writer is bringing to HBO a movie inspired by Spector's life that imagines his relationship with the attorney who defended him against charges of killing actress Lana Clarkson in Los Angeles in 2003.

The film, "Phil Spector," written and directed by Mamet, stars Al Pacino as the music producer and Helen Mirren as his attorney.

When Mamet's agent urged him to watch a documentary about Spector, the playwright said he felt he already knew enough about the eccentric producer who sported wild hair and was found guilty of murder.

"You start out saying this guy's a freak," Mamet told reporters at a Television Critics Association meeting on Friday.

Learning more about Spector, "you start to think, how could I be so prejudiced? The guy sounds brilliant."

"Then you say, maybe he's not guilty," Mamet said.

In the TV film that debuts March 24 on Time Warner Inc-owned HBO, Mirren plays Linda Kenney Baden, who defended Spector in his first murder trial that ended in a mistrial with jurors deadlocked. He was convicted in a second trial in 2009 and is serving a sentence of 19 years to life.

Spector, now 73, revolutionized pop music in the 1960s with his layered "Wall of Sound" production techniques, working with the Beatles, the Ronettes, Cher and Leonard Cohen at the height of his fame. But for years before the trial, he had lived as a virtual recluse in a mock castle in suburban Los Angeles.

WORK OF FICTION

The HBO film starts with a disclaimer saying it is a work of fiction "inspired by actual persons in a trial, but it is neither an attempt to depict the actual persons, nor to comment upon the outcome."

It tells how Baden became intrigued by Spector and the challenges of defending him. She considers how to raise reasonable doubt in the minds of the jury while the defense team wrestles with whether Spector should take the stand.

As his attorneys consider that Spector might hurt his own cause, Spector reminds them of his accomplishments. In one scene, he tells Baden: "The first time you got felt up, guess what? You were listening to one of my songs."

The real-life Baden told reporters on Friday that, as Spector's attorney, she couldn't tell Mamet about any conversations with her client. Instead, they were left to the playwright's imagination.

Baden said she felt the forensic evidence against Spector did not prove he killed Clarkson, who was found shot to death in the foyer of Spector's home hours after the pair met in a nightclub. Spector denied murdering Clarkson but did not testify at either trial.

Pacino said he didn't try to perfectly mimic the real-life music producer or meet with him, though he did watch video footage of his statements around the time of the murder trial.

"I would sit for hours just looking at Phil talking about things," said Pacino, speaking via satellite from New York.

Mirren said on Friday her biggest challenge was finding the right tone to play a character in the unconventional world that Spector seemed to inhabit.

"It's like a strange dream you are having," Mirren said. "The nature of Phil Spector and the life that he lived encouraged that. He seemed to live in a permanent dream."

(Reporting By Lisa Richwine, editing by Jill Serjeant and Philip Barbara)