Philip Seymour Hoffman, the stage and screen actor who progressed from scene-stealing supporting roles to an Oscar-winning portrayal of writer Truman Capote in “Capote,” has died. He was 46.

Mr. Hoffman was found dead in his apartment in Lower Manhattan shortly before noon Sunday, and his death is being investigated as a possible drug overdose, said Detective James Duffy, a spokesman for the New York Police Department. The New York City medical examiner’s office is expected to perform an autopsy as early as Monday, and that examination will include a toxicology report.

Police said they received a 911 call about 11:36 a.m. Sunday. When they arrived at the apartment, they found Mr. Hoffman unconscious and unresponsive on the floor of a bathroom. According to an unnamed police official who was not authorized to comment publicly, Mr. Hoffman was found with a needle in his arm and several bags of what appeared to be heroin. The official said Mr. Hoffman was supposed to meet a colleague Sunday morning and did not show. An associate went to his home and found him there. Police were still at the scene Sunday afternoon.

In interviews, Mr. Hoffman acknowledged a history of drug abuse.

“I got sober when I was 22 years old” and went into a drug rehabilitation program at the time, Mr. Hoffman told CBS’s “60 Minutes” in 2006. Asked whether he abused drugs or alcohol, Mr. Hoffman said: “It was all that stuff. Yeah. It was anything I could get my hands on. Yeah. I liked it all.”

Mr. Hoffman went on to say in the interview: “I have so much empathy for these young actors that are 19 and all of a sudden they’re beautiful and famous and rich,” Hoffman said. “I’m like, ‘Oh my God. I’d be dead.’ You know what I mean? I’d be 19, beautiful, famous and rich. That would be it. I think back at that time. I think if I had the money, that kind of money and stuff. So, yeah [I would have died].”

In other interviews, he indicated that he had remained clean for more than two decades before relapsing in 2012, when he again entered a drug rehabilitation program, according to published reports.

Mr. Hoffman, who specialized in off-kilter roles, won the best-actor Oscar for his 2005 portrayal of Capote in the biographical film that chronicled the writer’s research — and ethical transgressions — for the nonfiction crime novel “In Cold Blood.”

It was one of four performances that earned Mr. Hoffman an Oscar nomination. He was nominated for best supporting actor three times: for playing a CIA agent in “Charlie Wilson’s War,” an abusive priest in “Doubt” and Lancaster Dodd, a character loosely based on Church of Scientology founder L. Ron Hubbard, in “The Master.”

His work in “The Master,” released in 2012, was for director Paul Thomas Anderson. In total, the actor appeared in five Anderson films — all but one of the director’s six feature-length films.

“He was an extraordinary actor with tremendous range and the gift of fully and deeply realizing his amazing characters in films from ‘Magnolia’ and ‘Capote’ to ‘Charlie Wilson’s War’ and ‘Doubt,’ ” said Ken Howard, president of the Screen Actors Guild-American Federation of Television and Radio Artists. “He was such a great talent and his loss is just deeply sad. On behalf of his fellow actors and all members of SAG-AFTRA, our condolences go out to his family and friends.”

Mr. Hoffman got his start in Hollywood playing supporting roles in several movies in the 1990s, including “Twister,” “Patch Adams” and “Magnolia.” His breakthrough roles came as a gay member of a porn film crew in Anderson’s “Boogie Nights” and as an obscene phone caller in director Todd Solondz’s “Happiness.”

Most recently, Mr. Hoffman played Plutarch Heavensbee in the “Hunger Games” movies and appeared at the Sundance Film Festival last month to talk about his role in the film “God’s Pocket,” slated for release later this year.

In many of his performances, Hoffman’s characters bordered on either the manic or depressive, but he brought a depth and intellectual honesty to each of them beyond the lines of the script.

In his starring roles, his characters often took a darker course, in many cases being the antihero.

“Hoffman isn’t someone we want to be,” Claire Dederer wrote of Mr. Hoffman’s roles in Salon. “He’s someone we want to be better than. Here is an actor whose entire oeuvre can be described in one sentence: ‘At least I’m not that guy.’ ”

On Broadway, Mr. Hoffman was nominated three times for a Tony award, including for his portrayal of the worn traveling salesman Willy Loman in an acclaimed 2012 revival of Arthur Miller’s “Death of a Salesman.” His other Broadway roles included the oldest son of the Tyrone family in a 2003 production of Eugene O’Neill’s “Long Day’s Journey Into Night.”

Mr. Hoffman, who frequently dyed his hair and lost or gained weight for parts, was known for a sometimes painful dedication to his craft.

“With Capote, the part required me to be a little unbalanced, and that wasn’t really good for my mental health,” he told the New York Times in 2008. “It was also a technically difficult part. Because I was holding my body in a way it doesn’t want to be held and because I was speaking in a voice that my vocal cords did not want to do, I had to stay in character all day.

“Otherwise, I would give my body the chance to bail on me.”

Philip Seymour Hoffman was born July 23, 1967, in Fairport, N.Y., near Rochester. His father worked for Xerox and his mother was a lawyer, civil rights activist and, later, a family-court judge. His parents divorced while he was in his teens.

Mr. Hoffman wrestled in high school until a neck injury forced him to quit contact sports. He discovered acting almost by accident when he followed a young woman — one he had a crush on — to an audition and wound up joining the school’s drama club.

At 17, he was selected for New York State Summer School of the Arts in Saratoga Springs where he met director Bennett Miller and writer Dan Futterman, who later worked with him on “Capote.” He received a bachelor’s degree in drama from New York University.

Survivors include his partner of 15 years, Mimi O’Donnell; three children; two sisters; and his older brother, screenwriter and director Gordy Hoffman.

“We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone,” Mr. Hoffman’s family said in a statement. They asked that the public keep Mr. Hoffman “in your thoughts and prayers.”

Mr. Hoffman fully immersed himself in his craft and took pride in its creative challenge. “In my mid-20s, an actor told me, ‘Acting ain’t no puzzle,’ Mr. Hoffman once said. “I thought: ‘Ain’t no puzzle?’ You must be bad! You must be really bad, because it is a puzzle. . . . You start stabbing out, and you make a mistake, and it’s not right, and then you try again and again.

“The key is you have to commit. And that’s hard because you have to find what it is you are committing to.”

Greg Miller and Eddy Palanzo contributed to this report.