In a development a Hollywood scriptwriter would admire, a former patient of the slain Dr. Herman Tarnower took the stand in the Jean Harris murder trial today and said she had overheard an angry conversation between Harris and Tarnower the day Harris shot her lover dead.

Naturally, the patient -- a lady of many years and much propriety -- was a trifle embarrassed explaining this. She made a point of telling the jury she was sitting on the examining table in the doctor's office when the telephone rang, and she certainly had no intention of listening but had no choice.

"It was approximately 10 a.m.," began Juanita Edwards, testifying for the prosecution. "The doctor had just started taking my pulse when the telephone rang -- it was perhaps five feet away. . . . He said, 'I'll take it in my office' and placed the receiver in a bracket beside the phone, but it did not sever the connection. . . .

"I remained seated on the examining table and I soon became aware of the sound of muffled voices," Edwards continued, torn, as a lady will be, between admitting incorrect behavior and delivering some choice gossip. "All of a sudden I heard a voice I recognized as Dr. Tarnower, very loud and very angry. . . . 'Goddammit, Jean, I want you to stop bothering me,' he said. . . ."

There was more, the witness testified -- angry voices, one obviously a woman's, muffled voices. Then two more sentences she could discern, both in Tarnower's voice: "You've lied and you've cheated," she heard. And then, after another muffled exchange, "But you're going to inherit $240,000."

Edwards' testimony today contradicted testimony given earlier by the defendant, the former headmistress of the Madeira School in McLean, Va., who is now in the 12th week of her trial for second-degree murder in the slaying of Tarnower, her lover for 14 years, in the bedroom of his Westchester County, N.Y., home.

The prosecution, claiming that Tarnower had been involved with his assistant, Lynne Tryforos, says that Harris shot Tarnower intentionally, in a jealous rage. They also claim that Tarnower, the day of the shooting, told Harris that he no longer wanted to see her and had told her he had proposed marriage to Tryforos.

"Isn't it a fact that March 10, 1980, Dr. Tarnower told you you had lied and cheated?" prosecutor George Bolen had thundered at Harris on her last day on the witness stand last week. "Isn't it a fact that he told you, the morning of March 10, that you were -- quote -- going to inherit $240,000?"

Harris denied that, and also denied that Tarnower, in a phone conversation, had ever said to her, "Goddamit, Jean, stop bothering me!" She was, moreover, shocked by the accusations. "How long can this go on?" she asked the judge.

So the testimony today could severely hurt Harris. The witness, a gray-haired Westchester matron, was nothing if not credible. Her speech, though at first breathy and nervous, was refined. Her manner -- polite smile to the jury, nod to the courtroom -- was tea-party impeccable.

"Dr. Tarnower had been my husband's doctor for over 20 years; I saw him occasionally when my health warranted it," Edwards told the jury.

Her voice, as she recited the telephone call she had overheard, dropped and rose fine dramatic inflection.

Moreover, she had, when questioned, the proper air of contrite embarrassment regarding the phone call she had overheard.

"You didn't intend to overhear the conversation?" Bolen asked her at one point, courtly and diffident.

"I certainly didn't," the witness said, "I had no intention at all, but really I had no choice . . . . the voice was coming over so loudly, I couldn't help it, I heard. . . ."

The courtliness disappeared when defense attorney Joel Aurnou began cross-examining the witness. He paused at the witness stand before beginning his questioning. He turned, briefly, and gave the courtroom -- but not the jury -- his best fox-about-to-devour-Westchester-hen smile.

Then he began: When had the witness first seen this phone over which a conversation had been overheard? When had she last seen this phone? How did she know the phone had not been changed in the interim? How was it possible she had heard the call in the first place -- wasn't there a hold button or some such on the phone?

The witness was irritated but not shaken. She reprimanded the defense lawyer in a strong voice, as a teacher will reprimand a student who is simply not getting the point.

"Where is the button? Button, button, who's got the button? I don't know where the button is. Why are you dwelling on this button?" she asked.

The defense tried a number of other tactics; trying to shake the witness on the time she said the doctor had received the call; chiding her for listening to the call, rather than hanging up the phone. He suggested that the woman's voice on the end of the line was not heard because it was quiet, not angry; he suggested, in so many words, that part of the conversation the witness had overheard might not have been directed at Harris.

"Are you aware that the only family who will inherit $240,000 was Mrs. Tryforos'?" he said.

He cross-examined the woman for perhaps a half an hour.

When it was over, the prosecutor had only one question to repeat.

"Whose name did you hear the doctor use in the conversation?" he asked. "Jean," said the witness.