NEW YORK (Reuters) - Joaquin Phoenix knew he had to star in Spike Jonze's latest film Her, an unconventional romance that explores ideas about intimacy, isolation, relationships and technology, because it wasn't a typical love story.

Phoenix's depressed writer character finds love in the film, set in a futuristic Los Angeles, but the object of his affection isn't a co-worker or the girl next door. It's an intuitive, voice-controlled, computer operating system named Samantha.

"I thought it was really interesting and unique, that there was this great emotional center you could grab onto, that it wasn't too esoteric," Phoenix told Reuters ahead of the film's opening in U.S. theaters on Wednesday.

"There are these great big ideas and themes to explore but it felt totally accessible."

Phoenix, a triple Oscar nominee for his roles in Gladiator, The Master and Walk The Line, which also earned him a best actor Golden Globe in 2006, plays Theodore Twombly, a writer for BeautifulHandwrittenLetters.com.

By day he dictates affectionate letters that are transcribed by a computer into "handwritten" notes for clients to their loved ones. In the evening he plays a large video game alone in his high-rise, sparsely furnished apartment.

The National Board of Review named Her best picture of 2013, calling it "an outstanding achievement that is sure to become a new classic," and awarded Jonze its best director award.

Phoenix, 39, nabbed his fourth Golden Globe nomination for his role in Her, while the film won Globe nominations for best picture in a comedy or musical and best screenplay.

"In a tender about-face from his fearsome performance in The Master, Phoenix here is enchantingly open, vulnerable, sweet-natured and yearning for emotional completion," The Hollywood Reporter said about his performance.

The New York Post praised Jonze, who wrote the screenplay and is known for pushing cinematic boundaries since his off-the-wall 1999 film Being John Malkovich.

"Jonze seems to be heading for a far quirkier ending than the one he actually delivers, but he does tap into the zeitgeist with his unlikely romantic fable," it said.

LOVE IN THE VIRTUAL WORLD

Theodore is lonely, sad and in the throes of a painful divorce from his wife, a neuroscientist played by Oscar-nominee Rooney Mara until he falls for Samantha, voiced by actress Scarlett Johansson.

"I think, honestly, it was more difficult what she had to do in some ways," Phoenix said about Johansson.

The actress is never seen on screen and conveys Samantha's wit, intelligence and a range of emotions through her deep, throaty voice.

"As an actor, I think it is always nice to have access to tangible things. I love wardrobe and I love props and I love the environment I am going to be in, and to just be in a recording booth and to try to generate that emotion is very difficult. She had the hardest job," he added.

As their relationship evolves, so does Samantha. She organizes Theodore's emails, jokes and empathizes with him, composes music, and collates the letters he has written for his clients and submits them to a publisher to be released as a book.

Before long Theodore is smitten with the disembodied Samantha, whom he connects and communicates with through an earpiece, taking her on a mountain holiday, to the beach and on a double date with another couple.

Samantha even convinces Theodore to meet with a surrogate who can provide the physical aspects of the relationship that she can't.

Olivia Wilde, on screen in this year's Formula One racing film Rush, makes an appearance as a blind date for Theodore. The film also reunites four-time Oscar nominee Amy Adams with Phoenix. They two worked together in The Master.

Adams plays Theodore's best friend Amy, a woman who is going through her own marital breakup and can sympathize with his predicament and his dependence on technology.

"I loved the relationships Spike developed in the movie and how each of them was so specific," said Adams. "It was a story that was so much bigger than a love story. It was just a story about the human experience."

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Her, Joaquin Phoenix