The new Blade Runner, possibly the most feverishly anticipated film ever, is 65 days into shooting. Another reporter and I have slipped out of the tent where we are supposed to be watching proceedings on a viewing monitor to stand out in the cold on the edge of the tank and watch the real thing happening. It's so vast, so dark, so wet: actually, it's thrilling. The local Hungarian publicity assistants come out every couple of minutes and ask us would we not like to come back into the tent rather than stand here in a couple of centimetres of water. No, we wouldn't.

Here's what we can see. There is a car "crashed" on a sea-wall constructed on the edge of a tank big enough to look like the sea, with three rigs under the water surface creating waves, two cranes on dry land that can swing the car around and a vast scaffold rising out of the water with wings like a crop-duster that intermittently sprays rainstorms over two people grappling on the levee. Roger Deakins, the director of photography and an acknowledged genius, has lit the scene apparently only from within the car and a semi-circle of strip lights at ground level, which means that the pugilists move in and out of visibility, one moment in a blare of yellow light and the next plunged into the black.

We can only just make them out, to be honest; we're a good hundred metres away. As far as I can tell, there are stunt doubles who do the real fighting and roll into the water. Then they move off and Ryan Gosling, face bloodied, swaps feinting blows with newcomer Hoeks, whose strength and impeccably coiffed hair recommend her as a replicant. Filming this fight, we are told, will take three nights.

It must be getting cold up there; between takes, Gosling throws himself down on the cement and does some speedy push-ups while Hoeks trots with the briskness of a harness racer, ponytail swinging. Another wave, generated by a contraption of rotating buckets that could have been designed by Heath Robinson, surges over the car. Whoosh! The next level of wrangler comes and tells us we should go back to the tent right now. We won't and we don't. This is the new Blade Runner. We're not missing this.

Ridley Scott's Blade Runner, released in 1982, now ranks alongside Stanley Kubrick's 2001 as one of the two greatest science fiction films of all time. Opening with a sweeping shot across a glinting, glassy cityscape dominated by a vast advertising screen, fireballs of filth pumping through a fug of pollution and a crowd of doomed, desperate and deformed people scurrying around hundreds of metres below, it remains an unmatched vision of a near-future hell.

The story was simple enough. World-weary policeman Rick Deckard, played by Harrison Ford, pursues four "replicants" or humanoid robots that have gone rogue and returned from the space station where they are mechanical slaves to Earth, home of their original manufacturer, to demand longer, human-style lives. All replicants are retired after four years; they come with a selection of supplied memories and if they function for too long, they can also develop independent feelings. The escaped four want freedom. Deckard's job is to "retire" – read "murder" – them.

Based on Philip K. Dick's 1968 novel Do Androids Dream of Electric Sheep?, the film raised intricate questions about identity, memory and mortality: whether, for example, we are all just the sum of our pasts. It is hard to believe now that Blade Runner was initially a critical failure that also did badly at the box office, given that we have been chewing over it in all its official versions – there have been five – ever since.

The big question that has preoccupied the fanboys is whether Deckard himself is a replicant. It is this question that invests the current film with such urgency: surely, finally, this question will be answered. Ridley Scott, who directed the original and returns as executive producer this time around, said from the beginning that Deckard obviously was a Nexus 6, something he maintains to this day; Ford has always insisted he's human; Hampton Fancher, the original scriptwriter and co-writer of the new film, has hinted that he might be a replicant after all.

Comic-Con's kids and all their cohorts clearly need to know. But in what is surely a unique convergence of interests, the same issue is central for the numerous academics – not just film theorists, but philosophy and political science boffins – who have made Blade Runner the subject of endless critical thought. Because if the faltering, conflicted Deckard is not human, what does human mean?

We have no idea what the story in the new film will be. The trailers released since the set visit a year ago have given some hints. On set, we visit a bleak prairie house that the trailer reveals is a refuge from the authorities for a character played by Dave Bautista. The trailer also features Robin Wright as a government authority figure, saying (tantalisingly, given our current real-world situation) that "the world is built on a wall that separates kind – tell either side there's no wall and you've bought a war".

Jared Leto is the bad guy: a replicant manufacturer who declares that every great society required a slave class. It's not a great deal to go on. On set, the producers hold a press conference at which nobody can reveal anything. "You'll have to excuse me, we've been sworn to secrecy for years," says Gosling, stumped by a "how's it going?" question. "I have no idea how to approach not talking about it."

At least there are fewer night shoots this time around, says Ford. "Of the 50-day shooting schedule on the original film, 35 were nights," he says. "Which is a brutal regimen." Ford's fury at the end of that 1982 shoot has passed into legend, along with his terse dismissal of the film itself – "It's a film about whether you can have a meaningful relationship with your toaster" – but he's here anyway, playing Deckard 35 years on.

"Ridley and I have long since made our peace with each other," he says gruffly. "Whatever the circumstances were during the original film, I have great respect for Ridley and admiration for his work." Ridley Scott is not here, but his name is over everything: the script for which he is credited as story developer, the producer credits, people's minds. We are all replicants here, holding on to a firmly implanted photographic memory.

When Alcon Films producers Andrew Kosove and Broderick Johnson bought the rights to make a Blade Runner sequel in 2010, they imagined that Scott would direct it himself. "The first thing we did was call Ridley," says Kosove. "He wanted to be involved. Blade Runner was never intended to be a one-off, but because of the contentious nature of the first production between Ridley and the financiers, nobody was talking to anyone." Scott's Blade Runner went way over time and over budget by a then-stupendous $30 million, after which the studio re-cut the film, added a new, upbeat ending and insisted Ford record a voice-over, which he did through twice-gritted teeth. "The acrimony was so great. It was like the Middle East," says Kosove.

Alcon's initial proposal was to make a prequel that would show the apocalypse that resulted in Blade Runner's despoiled world. "But Ridley and Hampton got into a room and started rehashing ideas and came up with this amazing central concept hinted at in the first film, something nobody has ever talked about," says Kosove. "It was like discovering a key to a bank vault; we're very fortunate to have found the key that would bring them back together." Scott was busy with Prometheus, however, so he couldn't direct. The job passed to Quebecois director Denis Villeneuve, whose earlier films include Enemy, Sicario and last year's sci-fi revelation, Arrival.

Where Scott is abrasive, Villeneuve is emollient, a mix of Gallic charm and Canadian niceness. "Denis is a very different kind of director from ... what was his name?... Ridley," growls Ford. "He is very direct and straightforward with the actors on the set. He either deeply, deeply loves something or thinks it is dog poo and it becomes almost immediately clear. I have had a wonderful time working with him."

Villeneuve is too busy to do an interview, we are told, but he makes a point of leaving his impossibly complicated fight sequence to have a word. Recognising me from a recent interview, he gives my arm a companionable squeeze. That's it, I think: permission to stand here and get as wet as I want.

Villeneuve's line is that this is an indie film with a non-indie budget. "This is a very special set – I've never had this amount of toys. It's like Christmas," he says. The first Blade Runner, he has said on more than one occasion, was the film that made him want to be a director in the first place. So is he feeling a certain pressure?

"I had pressure earlier in the process, when I agreed to do it. Then I met Ridley Scott and I met Hampton Fancher, who gave me a lot of advice and comforted me a lot. But the pressure I cannot think about, because I would just run and hide under my bed."

The original Blade Runner was set in 2019. The glass towers and legions of indigent street dwellers imagined by Fancher, designer Syd Mead and effects whiz Douglas Trumbull – the two design giants of the pre-CGI era – have come to pass, even if we have yet to deal with flying cars or "skin-job" robots. Technology in their own business moved on, however, in ways they couldn't have imagined. Villeneuve is keen to keep to the spirit of the original by creating most of his effects in the camera.

The dominant palette is decidedly brown. "We worked with colour a lot, even though it seems there is no colour," says costume designer Renee April. "One thing Denis and Roger have done is make this dystopia, which is horrible, then make it look beautiful. It took a while to find cohesion, because the costumes had to go with the set, the lighting, the feeling of it – how sad or dirty or crazy it would be. Finally, we made a ton of big overcoats."

Props master Douglas Harlocker shows us around a collection of guns and the so-called "memory maker", a device that looks like a steampunk camera lens. A dog-like construction of scrap iron with a head like a scanner is apparently designed to sniff out replicants. Whoops, we're not supposed to know that. "Our inspiration comes largely from the director's cut, but also from other versions of the first Blade Runner," Harlocker says. "And from things they had in their heads but never used."

In designer Dennis Gassner's office, we are surrounded by drawings and photographs of cities in fog. "I asked Denis 'can you give me one or two words?' and he said 'I like chaos. We live in chaos today and we have to deal with it'," says Gassner. There will also be extensive use of models. "To have something to light is incredibly helpful," he says. "I love miniatures. I used miniatures to build the whole of New York City in The Hudsucker Proxy. It's interesting what sci-fi is going back to. It's all about tone."

We live in a cinematic age dominated by action films and, more particularly, by superhero action films: a long way, in other words, from Ridley Scott's Blade Runner with its ambiguities, moral twists and bottomless philosophical potholes. The trailers released so far by Warner Bros begin with Ryan Gosling pointing his futuristic revolver at us, followed by a lot of fighting. Blade Runner 2049 has to offer something more complicated than bad guys smashing through a skyscraper window, however, if it is to do anything like justice to the original. Denis Villeneuve can do that. He doesn't have final cut, but surely nobody would try to dilute or tamper with Blade Runner again. They have to deliver something extraordinary. And this time, we'll be ready for it.

Blade Runner 2049 opens on October 5.