At the time Kathleen Durst vanished in 1982, Ms. Berman served as a shield between Mr. Durst and tabloid reporters eager to discover what had happened.

Ms. Obst testified that Ms. Berman had revealed to her that she had tried to get the police off “Bob’s back” by making a phone call to Albert Einstein College of Medicine in the Bronx, posing as Mr. Durst’s wife, who was a student there. The phone call, prosecutors contend, was designed to make the authorities believe that Mrs. Durst was still alive after she left the couple’s home, 50 miles north of Manhattan.

Over the years, Ms. Obst said, she had forgotten the conversation. But “The Jinx,” a documentary in which she appeared, brought it all back. Still, she did not immediately go to the police. “I was afraid of the defendant,” Ms. Obst told the court. “He kills witnesses.”

Ultimately, Ms. Obst did contact John Lewin, the deputy district attorney in Los Angeles who is leading the prosecution of Mr. Durst, out of a desire to “do the right thing,” she said, and “to do the right thing by Susan.”

Ms. Obst was one of a string of witnesses called over three days by the prosecution in what are known as conditional hearings, in which testimony is taken from witnesses age 65 or older who could die or be killed before a trial, which, in this case, is likely to take place sometime next year. Ms. Obst was the second “mystery witness” to appear in the hearings — her name was not publicly revealed until she took the stand.