When Blanchard Ryan signed up for 'Open Water' she got a lot more than she bargained for - real live sharks, writes Donald Clarke.

When Blanchard Ryan, a 37-year-old actress whose CV then featured little more than an undistinguished string of guest appearances, was cast in the low-cost diving drama Open Water, all she was hoping for was a few good shots to add to her professional showreel.

Chris Kentis and Laura Lau, the husband-and-wife team behind the picture, were an unknown entity and, with a budget of only €100,000, Open Water would, she guessed, not stand much of a chance in a summer season dominated by heavily bankrolled behemoths. Things worked out rather differently.

"Just yesterday Chris and Laura and I were having lunch," she says. "And suddenly we all said as one: 'My God. We are in London promoting this film. How on earth did that happen?' "

The success of Open Water is indeed one of the more interesting phenomena of the season. Based very loosely on true events, the film tells the story of a holidaying couple who, victims of a terrible mishap, find themselves abandoned at sea by the boat that has taken them out diving. As time passes they realise that they have started to drift. Then sharks begin to circle. The distributors have, with more than usual insistence, asked us to reveal no more of the plot.

The film's main selling point is its authenticity. The sharks weaving between Ryan and her chiselled co-star, Daniel Travis, are not made of rubber; they are the real thing. I wonder how clearly the actress understood what would later be asked of her when she attended the audition.

"I blame my agent. She said there would be some nudity and I would have to swim with sharks. We always thought that there was going to be a shark or two. But when we got out there, there were dozens of them. If anything we had too many sharks for our needs."

Lau and Kentis shot the shark sequences over an arduous two days in the Bahamas. Kentis (credited as writer and director, though he and Lau, nominally the producer, share all film-making duties) had heard encouraging things about the excursions run by the renowned shark expert Stuart Cove. Without identifying himself, Kentis went out on one of Cove's jaunts.

"We got out to open water and suddenly these sharks were everywhere," Kentis says. "But all these tourists were going in among them and people were dropping their eight-year-old children in there on top of them. I felt if he is confident enough to do this with totally untrained people then he could surely do it safely with us."

With admirable fortitude, Travis and Ryan, who had only limited diving experience when they were cast, spent those two long days floating amid featureless nowhere while Cove and his colleagues shovelled buckets of tuna about them and Kentis paddled around clutching his digital video camera. Kentis explains that the sharks in question had become used to this particular vessel and were comfortable among swimmers, but it still doesn't sound awfully safe.

"The truth is it would be foolish to say you are in no danger when you are talking about 800-pound animals with sharp teeth. However, we weren't taking any chances. We were all wearing this chain mail beneath our wetsuits, though I eventually ended up taking mine off, because it made operating the camera difficult. Daniel and Laura and I all found it very exciting. I think that was more difficult for Blanchard."

Ryan is keen to make clear that she was never asked to do anything that had not been agreed well in advance. But she confirms that she was by far the least comfortable in the water. "The sharks don't get out of your way," she explains. "And it was the most terrifying thing I have ever done in my life. I remember it being terrifying from morning to night both days. But there is footage of me joking and seeming quite calm and relaxed. I suppose as a human being you do reach a stage of relaxation and acceptance. I would then think I was being a big nancy and would relax. But all it would take would be a tail to whack against me and I would scream."

As it happens, the only serious bite she received was from a barracuda. "Yeah, that is my barroom story," she says, showing me a scar on her knuckle. "I could have lost a hand, but I didn't know what it was at first. I thought maybe I had rubbed up against some fire coral. When I found out what it was I just wanted to make sure they had it on tape." Sadly, panicked at Ryan's distress, Kentis had dropped the camera.

WHEN DISCUSSING OPEN Water it is tempting to focus exclusively on the sharks. The perennial sexiness of the aquatic predators and their singular role in the shooting process have encouraged the distributors to, rather disingenuously, sell the film as a combination of Jaws and The Blair Witch Project (boycott any media outlet so unimaginative as to use those precise words). But the sharks are only one element of the film's appeal. With fairly few popcorn-in-the-air shocks, Open Water feeds on creeping despair rather than violent catastrophes.

Considering the weight that Lions Gate Films, its distributor, has thrown behind Open Water, I don't imagine that Kentis will complain too much about how the advertising has been handled. But it does give a misleading impression, doesn't it? "I definitely have concerns about that," he says. "You have to leave it with the marketing people, but it does set up certain expectations. We didn't set out to make a genre film or a shark film or a horror film. Sharks were always going to be one element of the story. We would have to deal with that element. If you go looking for Jaws you will be disappointed. I love Jaws, but it is seeking to do different things."

Kentis and Lau, whose first film was the barely seen Grind, which featured Billy Crudup in his first leading role, conceived Open Water as their way of emulating the imaginative uses the Danish Dogme 95 movement had made of digital technology. "We had just had our first daughter, and the freedom that shooting in that way grants you really appealed to us," Kentis says.

One day, while reading the respected scuba journal Undercurrent, Kentis - like Lau, a very keen diver - came across the true story of a US couple who had been abandoned off the Great Barrier Reef. "We did some research and discovered there were other similar incidents," he says. "We didn't want to tell the story of the personal lives of the people in the story. What fascinated me was: what was it like out there? Hopefully, if we designed the film properly we would put audiences right there in the minds of these characters."

Lau and Kentis, who both emanate the cheery bumptiousness of people who like to get things done, never considered bringing their idea to a major studio. They always believed the low-budget aesthetic - keep crews tiny, move quickly - would be crucial to the success of the production. Open Water was made entirely with their own money and so involved no compromises to anything other than the demands of frugality.

ASIDE FROM RYAN'S squabble with the barracuda, the only major setback occurred when, only three days from the end of the shoot, Daniel Travis injured his knee playing volleyball. They were unable to get back in the water for 11 months. "Everything was ready to go, and I had to call Chris and tell him I was going into surgery," Travis says. "There was a very, very long pause, and then he took it quite well. And through the entire process that was the most unpleasant moment I went through."

When the movie was finally finished, Kentis and Lau felt confident they had something reasonably commercial on their hands. They were, nonetheless, surprised when Open Water was the first film sold to a distributor at this year's Sundance Film Festival. They were further dumfounded when Lions Gate announced it would be striking more prints of the film than anything else it had ever released.

"They said they were taking it wide," Kentis says. "They were talking 1,500 screens and we were like: wow! We knew they werehappy with the excellent press coverage, but then suddenly they were talking about 1,800 screens - and we were pinching ourselves."

Although the film has not made an enormous splash at the box office - it is certainly no Blair Witch Project in that regard - it has done enough to single out its makers as film-makers to watch. They have received a predictable series of shark and diving scripts in the post but are keen to move in an entirely different direction. Whereas the attention of agents and producers is welcome, the couple seem most delighted by the approval of the diving community. Indeed, these fearless shark wranglers admit that showing the film to a specialist audience was the most frightening part of the whole experience. The response was positive, and approving e-mails have continued to stream in.

Kentis explains: "What was funny was that at first, because we said it was based on a true story, we used to get these e-mails saying: 'Yes, I was left behind like that, but that there didn't happen and this here didn't happen.' I had to keep politely replying: 'I'm sorry. The film really isn't about you.' "

Open Water is on general release