Schackman, who says he and Simone were like "brother and sister" for nearly five decades until her death in 2003, tells PEOPLE he believes Saldana "did an admirable job" playing the iconic singer.
Saldana's casting in the role has sparked controversy, with many critics questioning the use of dark makeup and a prosthetic nose to make the actress more closely resemble her character.
"Of course she did not look like Nina in really any way," Schackman concedes, while noting that 'Diana bore little resemblance to Billie Holiday when she played the singer in Lady Sings the Blues (1972). "There's a certain amount that the movie goer, fans and even myself have to accept and try to keep an open mind about," he adds.
"I think that Zoë studied Nina and did a studied and sensitive portrayal of the aspects that [writer and director] Cynthia Mort gave her to work with. She couldn’t work with what she didn’t have."
For instance, Schackman points out that Nina is not a composite of Simone's life, but merely an interpretation. The film covers only a small sample of Simone's complicated life, focusing on her relationship with Clifton Henderson (played by David Oyelowo).
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"I immediately had problem with [Mort's] depiction and portrayal of Clifton Henderson as being a romantic interest and a love story for Nina, which he was not," he says. "Clifton was outwardly gay and a pretty serious alcoholic, so that aspect I had, and still have a problem with."
In the interest of bringing some authenticity to the film, Schackman focused on getting the music right. "I said if I'm going to contribute to this movie, the music has to be authentic." So at Mort's request, he brought the original Nina Simone band to Capital Records in Los Angeles to record the soundtrack with Saldana on vocals.
"We recorded 16 tracks exactly the way we would have played them with Nina to back Zoë up," Schackman explains. "I think she did a pretty decent job."
Part of Saldana's allure, according to the bandleader, was that her voice is completely unknown to the public. He feared that if a well-known artist, like Mary J. Blige (who was originally considered to play Nina in the film) performed the songs, audiences would recognize Blige's voice and get taken out of the story.
"Nobody could have done it, Mary J. couldn't have done it," Schackman argues. "If Mary J. Blige had stayed on and done the movie you would have heard Mary J. Blige singing Nina Simone, or any vocal star. In this case, with Zoë doing the vocals herself, you can't identify it with a famous music star because Zoë never sang [publicly]."
As for how Simone might feel about the movie, Schackman says "it's almost impossible for me to say because I never knew how Nina was going to feel about anything." He does think Simone would have liked one scene in particular, in which she pulls a knife on a French club goer for being rude. "Now that’s a part of Nina that's true," he says with a laugh.