Aaron Sorkin talked to a key person in Steve Jobs' life whom even Jobs' biographer couldn't get to - and it changed the whole script of his movie

Warning: Spoilers ahead

In the new movie "Steve Jobs," a major theme is Apple cofounder Steve Jobs' relationship with his daughter, Lisa Brennan-Jobs.

For Aaron Sorkin, the film's screenwriter, meeting Brennan-Jobs before writing the script was invaluable. And it's an interview even Walter Isaacson couldn't secure for his Jobs-approved biography.

"At first I didn't know what I was looking for," Sorkin told Business Insider, hours before the film premiered at the New York Film Festival. "Lisa didn't speak to Walter Isaacson when Walter was writing the book ["Steve Jobs"] because her father was alive at the time. But she was willing to speak to me. She was able to tell stories about her father that weren't necessarily flattering stories, but she would tell the story and then show me how you could see he really did love her."

Sorkin said hearing her tell those stories made him want to have a major part of the film be about the father-daughter relationship.

Brennan-Jobs is featured in all three parts of the film, which looks at the launch of the Macintosh, Jobs' NeXT, and the iMac. In a heartbreaking sequence in the first act, Jobs (played by Michael Fassbender) rants that Brennan-Jobs is not his daughter and that the Apple Lisa was not named after her.

Both "Steve Jobs," directed by Danny Boyle, and "Steve Jobs: The Man In The Machine," the recently released documentary by Alex Gibney, spend time looking at Jobs' relationship with his daughter.

When Jobs launched Apple, he denied he was Lisa's father, even when a court-ordered DNA test proved that he was.

Jobs gave only $500 a month to Lisa's mother, Chrisann Brennan, for child support - even though he was worth more than $225 million.

But, according to the Gibney documentary, by Jobs' death in 2011 the two had a better relationship.

Sorkin, however, did acknowledge to Business Insider that if Jobs were still alive, Sorkin would ask him to truthfully answer this question about his daughter:

"I would ask, 'Why do you pretend you didn't name the computer after Lisa?' I can't fathom," Sorkin said. "Any other father, if they hadn't named it after their daughter would lie and say they did. I just can't fathom it."

"Steve Jobs" opens in theaters on Friday.