The untitled television movie centres around Spector's (Pacino) 2003 trial for the murder of actress Lana Clarkson.
Mirren plays Linda Kenney-Baden, the attorney tasked with defending the famously volatile Spector.
The Oscar-winning Mirren praised Pacino's dedication to acting, adding that collaborating with the Scarface star was unforgettable.
"[It was] great, really great. Really inspiring and one of the highlights of my life to work opposite him. That was fantastic," Mirren told WENN. "[I was] very lucky."
The Spector biopic, which is written and directed by David Mamet, is scheduled to air in 2012.
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Watch a scene from Helen Mirren's new film The Debt below: