John W. Hinckley Jr. picked up ideas from defense psychiatrists, the news media and his own lawyers to gradually embellish his description of his mental state on the day he shot President Reagan, a prosecution psychiatrist testified yesterday.

Dr. Park Elliott Dietz told the jury that while he was sure no suggestions were made "willfully or maliciously," Hinckley drew upon ideas from these sources in the same way he had plagiarized college papers, imitated characters in books and movies and seized on other thoughts which he then claimed to be his own.

While Dietz testified, Hinckley sat at the defense table mugging and wagging a finger at nearby news reporters and artists. At one point, while the jury was out of the room and the lawyers were at the judge's bench, Hinckley put his necktie in his mouth and posed for several minutes, to the visible consternation of the deputy marshal seated behind him.

The jury, meanwhile, has appeared patient during Dietz's protracted testimony, which has consumed the past four days of Hinckley's trial. Judge Barrington D. Parker has interrupted the proceedings on a few occasions to call the courthouse equivalent of a seventh-inning stretch, allowing the jurors to stand briefly at their seats. Yesterday, as the afternoon session wore on, one juror sat comfortably filing her fingernails.

Dietz is the first of four psychiatric experts called to testify in the government's effort to prove Hinckley was sane when he wounded Reagan and three others outside the Washington Hilton Hotel more than a year ago.

During the trial, now in its sixth week, the jury has heard equally extensive testimony from three defense psychiatrists. All of them contended Hinckley suffered from a severe mental illness at the time of the shooting and should not be held criminally responsible for his acts.

Much of the defense testimony focused on Hinckley's voluminous writings in the years before he shot Reagan, including an extraordinary assortment of poetry that the defense psychiatrists suggested revealed Hinckley's mental illness. Yesterday, however, Dietz flatly discounted the value of that poetry, which he called fiction, as a means for judging Hinckley's state of mind.

"If we were to judge the mental state of an author by his poetry, our mental hospitals would be filled with the most distinguished poets in history," from e e cummings to William Shakespeare, Dietz told the jury.

Moreover, Dietz said, by the time he had seen copies of the poetry and went to interview Hinckley, defense psychiatrist Dr. William T. Carpenter "had already been there suggesting his psychiatric interpretations of these poems."

During cross-examination, defense lawyer Vincent J. Fuller reviewed with Dietz 10 examples of statements by Hinckley, recorded in Dietz's notes, which were not included in a final report submitted to the court by the prosecution psychiatrists. Dietz said the psychiatrist who wrote that section of the report would have to explain the deletions when he takes the witness stand.

Dietz told the jury that while Hinckley had imitated certain features of Travis Bickle in the movie "Taxi Driver," he suggested that Hinckley exaggerated his links to the movie after he read media accounts that detailed the similarities between Hinckley and Bickle.

The defense psychiatrists testified that Hinckley, distraught by the murder of John Lennon, began to assume characteristics of Lennon's killer, Mark Chapman. Dietz said Chapman's lawyers telephoned Hinckley's lawyers and discussed similarities in the cases. Dietz testified that Hinckley and his lawyers then discussed those parallels.

Later, Dietz acknowledged that evidence in the case showed that months before he shot Reagan, Hinckley purchased the same type weapon used by Chapman.

Defense psychiatrist Carpenter testified earlier that a tape recording Hinckley made on Dec. 31, 1980, was filled with themes of insanity, suicide, homicide and kidnaping. Dietz told the jury yesterday, however, that while the tape does imply thoughts of suicide, Hinckley has repeatedly stated that he was "drunk" during the monologue.

"Of course there are many people who cry in their beer," Dietz said.

Dietz, an assistant professor of psychiatry at the Harvard Medical School who specializes in examining criminal defendants for court trials, suggested that Carpenter inadvertently planted ideas in Hinckley's head because he has no experience in dealing with criminal cases.

Carpenter, who interviewed Hinckley for 45 hours, is the director of the Maryland Psychiatric Research Center and a well-known expert in schizophrenia, a severe mental illness characterized by extreme breaks with reality. All the defense psychiatrists testified that Hinckley suffered from schizophrenia.

Dietz conceded Hinckley's loneliness and grandiose ideas of self-importance would be consistent with that diagnosis of schizophrenia, but said neither feature was extreme enough to support the defense finding.

Carpenter earlier told the jury that Hinckley saw Reagan and his other victims as "bit players" in his effort to accomplish a union with actress Jodie Foster. Dietz said yesterday that Carpenter may have suggested that notion to Hinckley, but he did not elaborate.

Dietz said Hinckley told him that while he was in custody at Fort Meade, Md., he wrote a movie script about his life, "starring me," and also starring "Ronald Reagan, his wife Nancy . . . doctors, lawyers and hangers on." Dietz said Hinckley told him that in the movie's final scene he takes Foster away from Yale University "and this world forever."

Dietz said Hinckley told him: "The movie ain't over yet, folks."