Infamous

Truman Capote, known in New York City society for his wit and fashion flair as much as he is recognized in literary circles as the celebrated writer of Other Voices, Other Rooms and Breakfast at Tiffany's, reads a brief article about the murder of a farming family in Holcomb, Kansas, in the back pages of the New York Times of November 16, 1959.
Curious as to how the residents would react to a brutal massacre in their midst, the author and his friend, Harper Lee (Sandra Bullock), travel from New York to the rural Midwestern town, ostensibly so Capote can interview people for a magazine article. Once there, he realizes there might be enough material for what he eventually describes as a nonfiction novel.
Capote's dress and demeanor both amuse and dismay law enforcement officials. He allows the less ostentatious Lee to act as a buffer between himself and those whose trust he needs to gain in order to obtain as much background information as possible.
The Kansas Bureau of Investigation's lead detective on the case, Alvin Dewey (Jeff Daniels), has refused to cooperate with the writer. But when his starstruck wife Marie meets Capote in a grocery store, she invites him and Lee to Christmas dinner. He eventually wins over his host with personal anecdotes about Humphrey Bogart, John Huston, Frank Sinatra, and the like.
As a result, when ex-convicts Richard Hickock (Lee Pace) and Perry Smith (Daniel Craig) are apprehended in Las Vegas and extradited to Holcomb, permission is given to Capote to interview them in their cells. The two men are tried and found guilty, but a lengthy period of appeals begins. Capote's society and literary friends like Slim Keith and Babe Paley in New York press him for juicy gossip about the case and inquire when they can expect to read the book.
Capote forms an attachment to Smith. He empathizes with the convicted killer's unhappy childhood, and Smith's remorseful manner, genuine sincerity, and obvious intelligence impress him. The criminal's reciprocal feelings become evident, although Smith has difficulty dealing with his emotions. As soon as Smith learns that Truman plans to title his book In Cold Blood, which suggests the author thinks of him only as a merciless killer, he violently subdues Capote and nearly rapes him.
Smith steadfastly refuses to describe the night of the murders. This greatly angers Capote, who wants to hear details not only as a writer in search of the truth but as someone who finds it difficult to believe a loved one could be guilty of such a crime. Smith eventually acquiesces and discusses what transpired.
Capote then finds himself entangled in a personal and professional dilemma. As much as he wants Smith to be sentenced to life in prison, a death by hanging would provide a far more sensational ending for readers of his book. He begins to unravel psychologically as the legal appeals drag on, unable to complete his book without an ending. Lee, during the interim, writes To Kill a Mockingbird, which becomes a best-seller.
Years go by. Hickock and Smith finally exhaust all their options; they now ask that Capote be present at their April 14, 1965 execution. He complies reluctantly with their request. Afterward he learns Smith bequeathed his meager belongings to him, and among them he finds a charcoal sketch of him the killer had drawn.

Question: Why is Capote interviewing people ?
Answer: Magazine article.