The contemporary Western "Dark Blood" includes what is believed to be the last footage of Phoenix, who died aged 23 of a drug overdose just 10 days before he was meant to finish the shoot.
Dutch director George Sluizer, now 80, resumed work on the film in 2009 and narrates missing scenes aloud over still frames in the completed movie.
The picture, which runs for just under 90 minutes, tells the story of Boy, a grieving young widower with Native American roots who is living at a nuclear testing facility in the desert after his wife died of radiation exposure.
While waiting for the world to end, he picks up a feuding wealthy couple travelling through the same area (Jonathan Pryce and Judy Davis) when their Bentley breaks down.
Boy is drawn to the attractive wife, the former Las Vegas showgirl Buffy, and refuses to let the couple go, thinking he can convince her to join his one-man doomsday cult.
Pryce told a news conference in Berlin after a screening that drew a mixed reception that he had been impressed with Phoenix "and I love his memory."
"I found him a remarkable young man - I can't believe now looking back that he was only 23 at the time. It's that kind of old head on young shoulders," he said.
"But he was absolutely delightful, he was wonderful to work with, he was very committed, very serious about his work."
Sluizer said he had once joked with Phoenix, whom he called a "gentle actor", about whether he minded working with a director much older than himself.
"And he said: 'George, I respect people who are older than myself because they are wiser and more experienced'," he said.
The picture, shot in Utah and New Mexico, underwent an odyssey after Phoenix's death when an insurance company took possession of the unfinished footage.
In 1999, Sluizer bought the material to rescue it days before it was slated for destruction and said that after he had a near-fatal aneurysm, he decided he had to finish the project before it was too late.
But he said that beyond the footage that he was unable to shoot due to Phoenix's death, key reels had also gone missing in the intervening years.
"It made it necessary to rethink the story and what could be told to eventually make it understandable for an audience and pleasurable to watch," he said.
Phoenix shot to stardom with brooding performances in hits such as Rob Reiner's "Stand By Me" and as a narcoleptic gay hustler in Gus Van Sant's "My Own Private Idaho" in which he played opposite a young Keanu Reeves.
He was nominated for an Academy Award for the 1988 drama "Running on Empty."
His sudden death in 1993 of drug-induced heart failure outside Johnny Depp's Los Angeles club, the Viper Room, made Phoenix into a kind of James Dean of his generation.
Pryce said he believed he was trying to kick drugs at the end of his life.
"In all the weeks we were together in Utah - and I did spend every day almost all day with him - at no time did I experience him using drugs or abusing drugs in any way, shape or form," he said.
"I'm not a drug user myself but I'd have known. He was at a point in his life that he was very committed to not using drugs and it was apparent."
River was the older brother of other actor members of the Phoenix clan including Joaquin, who is up for an Oscar this year for his work in the Scientology saga "The Master."
Sluizer said the Phoenix family had not wanted to participate in the project, but that he spoke with his mother who "wishes the best for the film."
"Dark Blood" is screening out of competition at the 63rd Berlinale, which will award its Golden and Silver Bear top prizes on Saturday.