This is a story about death and about things left unfinished.

On October 31, 1993, at around one in the morning on the sidewalk outside the Viper Club, Los Angeles, River Phoenix began having convulsions. The actor, who was 23, died an hour later. Los Angeles County Coroner's Office revealed he had "acute multiple drug intoxication", including lethal levels of cocaine and morphine.

George Sluizer was sleeping when he got the call. He had seen Phoenix only a few hours earlier, leaving the hotel they were both staying at during the final days' shooting of the film Sluizer was directing.

"When I arrived at the hotel, he was driving away with a girlfriend and his brother Joaquin and two other guys in the car. I said, 'See you tomorrow' and he said 'See you tomorrow'. Well, that was the last thing. He went to the Viper Club and then you know what happened after that."

Sluizer is 81 years old and under a death sentence himself. In 2007 he suffered an aneurysm and was, he says, "condemned really by the medical staff" and given six months to live. He was hospitalised for a year and then had to undergo rehabilitation. His health is poor, is disabled and is in a wheelchair.

But on the phone from Nice, where he lives, the Dutch director sounds vibrant, even feisty ("You want me to finish a sentence?" he says when I ramble on too much). We are speaking because he is bringing Phoenix's final film, Dark Blood - which also stars Judy Davis and Jonathan Pryce - to the Glasgow Film Festival. It is an unfinished, mutilated movie, missing key scenes, but he has cut it together and narrates the missing sequences himself. The result is a skeleton of a movie, all the more frustrating because it is so close to being finished. But even in its partial state, you can get a sense of what we've lost.

That we have even got this is something of a miracle. Sluizer retrieved the film in 1999 from Los Angeles two days before it was going to be destroyed.

"It was going to be burnt two days later by the insurance company after it lost a lawsuit with the family of River Phoenix. I heard that from the loss adjuster. He offered me the key to the storage room, but then unfortunately could not find it at the last minute. So we just had to be clever; unlocking the storage room and getting the material out. We had to go fast, get it into a truck and then ship it to Europe."

That entailed moving 700 kilos of film, which worked out at, he says, 69% of the film. He didn't look at it for years until, recovering from the aneurysm, he thought that editing the material together would be one last thing he could do.

Sluizer was used to difficult shoots. He had worked on Werner Herzog's notorious film Fitzcarraldo, shot in the Amazon jungle. But Dark Blood, shot far from civilisation in the Utah desert, had its own problems. Mostly of the human kind.

"Judy Davis is not an easy lady," Sluizer says bluntly. "'I don't want to do this and I don't want to show my face on the left side,' or whatever. When you're in a bad mood, then the emotion becomes important, and if you don't want to do something then you get nasty to your director. I must say Judy, on the other hand, is a very special actress in the sense that the moment the camera turns and you say action, she becomes the character. And the moment you say cut, she can scream at you."

Davis and Phoenix didn't get along either, but Sluizer has nothing but good words to say about his doomed star. It had taken months to get Phoenix's agent to even send the actor the script.

"After four months the producer said to me 'Well, George, we can't get River. You have to choose another actor.' So I said 'If I have to, Johnny Depp.' But he was lazy. He had been used to working with Tim Burton and friends. He didn't like to get out of his friends' group. He kept saying 'I'll read it at the weekend.' The weekends went by and I think a month or two later my film Utz came out in Los Angeles, and I guess River's agent went to see it."

The script went to Phoenix, he asked to meet Sluizer, and then agreed to make the film the next day. The actor Sluizer met was, he says, "a very charismatic young man". They both travelled to Utah a few days before shooting started and went walking together - climbing hills and talking. "This was getting to know someone. It was a way for him to know who was this man who was going to bully him around later. And on my side, to know who River was."

What did he learn about Phoenix in those days together? "Well, obviously he's a fantastic liar, which he was very proud of, by the way. He was proud of lying to journalists."

For all the problems on set in Utah, the film, relocated to Los Angeles for interior shooting, was nearing its final days when Phoenix died. Were there indications that things were about to go wrong?

"The day before [Phoenix's death] was the only day we shot in Los Angeles in the studio. During the day he was, let's say, slightly stoned. I could notice that he could not distinguish distances adequately."

That said, Sluizer adds: "When we were shooting he was absolutely a full actor."

But, according to the director, Phoenix's death was completely unexpected. "That he used drugs, that was clear. But he did not use drugs while we were in Utah that I could see or notice in those six or seven weeks. The day before we left he said 'I'm going back to Los Angeles, a bad, bad town.' It didn't ring a bell in my head."

In the days and weeks after Phoenix's death, Sluizer gravitated between grief and anger. "I was furious at the fact that we could not finish the film, furious at River at the same time as crying about him dying. Furious that he f***ed up three years of effort."

As the film got tangled up in a legal dispute, Sluizer questioned whether he would ever want to make any more movies, "because I didn't want actors to die in things that I was involved in. Slowly you go back into your craft. But I was not interested for a very long time."

Two decades later, Dark Blood exists, like Sluizer, in a damaged state. It's a film that is a partial tribute to Phoenix's talents. And to the great American actress, Karen Black, who has a small role in the film and who sadly died last year.

It's an unfinished film that ends in death. On the screen and tragically, in real life too. Death is always an ending.

Dark Blood screens at GFT on February 24 as part of the Glasgow Film Festival. George Sluizer is hoping to attend.