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A veteran Hollywood producer took the stand in the Robert Durst case on Wednesday and gave prosecutors another piece of the puzzle they say will show that the real-estate heir killed his first wife and then a confidante he feared would implicate him.

Lynda Obst, who produced "Sleepless in Seattle" and "Interstellar," was the second so-called secret witness asked to testify at pre-trial hearings in Los Angeles. She called him a "scary person."

"I was afraid of the defendant," she testified. "He kills witnesses."

Lynda Obst in Beverly Hills, Calif., last year. Willy Sanjuan / Invision/AP file

Obst, who appeared in "The Jinx," the HBO series about Durst, was a close friend of Susan Berman, a writer who was murdered in 2000 — allegedly because she knew too much about the millionaire's involvement in the 1982 disappearance of his first wife, Kathie.

Under oath, Obst recalled a conversation she'd had with Berman about Kathie's disappearance. Berman said that at Durst's request she had posed as Kathie and called in sick to Albert Einstein Medical Center, where the missing woman was studying medicine.

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The detail is key because the prosecution contends that at the time of the call to Albert Einstein, Kathie was already dead. Convincing a jury that someone else made the call bolsters the district attorney's timeline of the events around her disappearance.

Real estate heir Robert Durst appears in a Los Angeles Superior Court Airport Branch for a pre-trial motions hearing on Friday, Jan. 6, 2017, in Los Angeles. Mark Boster / AP file

Under cross-examination, Obst, 67, was questioned about why she didn't tell anyone about the call at the time of Kathie's disappearance, after Berman's slaying, or even during her interview for "The Jinx."

"It would have been marvelous if I had remembered it," she said of her appearance on the HBO show, which examined Durst's links to Kathie's disappearance, Berman's murder and the dismemberment of an elderly neighbor in Texas.

"I did the interview cold."

It wasn't until she watched "The Jinx" and saw a reference to the mysterious call to Albert Einstein that she realized, with "heart racing," that she had potentially important information. She called the director, Andrew Jarecki, who told her to call the district attorney, which she did after much hesitation.

"I was reluctant to get involved," she said.

Robert Durst; Kathleen Durst Reuters file

She said that even though Durst is frail and behind bars, she worried about her safety.

"He has means, even from prison," she said.

And, she said, she had long suspected that he was behind the death of Berman.

"I suspected Mr. Durst," she said. "I just know that I did."

Durst, who is serving seven years on a federal gun charge, has pleaded not guilty to murdering Berman; he is not charged in connection with Kathie, who is presumed dead.

The judge has not yet decided if the jury will be allowed to hear testimony from Obst and other pre-trial witnesses who were brought to the stand so their testimony could be recorded in case they are not available during the trial.