Christopher Nolan is no stranger to difficult movie shoots, but with his war drama Dunkirk, there were times the director thought he might have bitten off more than he could chew.
This is because he had ambitiously decided to re-create the famous 1940 evacuation using real military aircraft, naval and civilian ships plus thousands of extras.
Upping the ante further, he opted to shoot using mostly large-format Imax cameras and did so on location in Britain, Holland as well as the beaches of Dunkirk itself - the port in northern France from where some 338,000 British and French soldiers had to be evacuated in 1940 after being trapped there by the German army.
This meant that on top of filming aerial combat sequences and marshalling a cast the size of an army, Nolan had to shoot complex scenes in the open water - something that has been the undoing of many a director.
While he had some knowledge of working with large numbers of extras and filming tricky aerial sequences, he had "absolutely no experience" with marine work, he tells The Straits Times.
"So being out there with what was the largest marine unit in movie history, so I'm told - it was more than 60 boats at one time - it was quite a thing to try and do," says Nolan, who also used real British Spitfire fighter planes and Messerschmitt German aircraft.
Speaking to the press in Los Angeles, he admits: "There were certainly moments where I thought, 'Okay, I wanted to do all this for real and all in camera - is that unrealistic, am I pushing things too far and being unreasonable?'"
Sensibly, he had turned to Oscar- winning film-makers Steven Spielberg and Ron Howard for tips on the water scenes.
Howard filmed the maritime drama In The Heart Of The Sea in 2015 and Spielberg made the iconic 1975 shark movie Jaws, after which he famously swore he would never shoot on water again.
Their advice to Nolan? "Well, they told me it's a nightmare and just be ready," he says, chuckling. "And they were right."
Both of them advised Nolan to use handheld cameras when shooting on boats - even though these were large, heavy Imax cameras - because the cameramen could then steady themselves against the movement of the vessels.
"And Ron very specifically told me there's no point going out on the open water at night because you can't light it. If you light it, it looks like you're in the studio anyway, so you might as well do tank work, which is the perfect thing I needed to hear because it's exactly right."
But that still left the many daytime scenes to figure out and, for that, Howard confirmed that large water tanks "weren't going to be useful because none of the tanks have usable horizons. So you really have to get out in open water if you're shooting a day scene".
Nolan, his cast and crew thus had to brave frigid, choppy waters for much of the shoot, and the actors often wore partial wetsuits under their costumes.
"But I had an amazing team around me and they really started to understand and promote the vision of what the film could be, and that was really rewarding," he says. "I think that helped maximise what the film could be."
A key member of that team is Nolan's wife, Ms Emma Thomas, 48, who has co-produced many of his movies.
She is often able to translate his vision - and exacting standards - for the rest of the crew. And she does not hesitate to speak up if she disagrees with him.
"I would say part of Emma's job is constructive scepticism," Nolan says with a smile.
Asked if they have any rules about not taking their work home, the couple look at each other and grin sheepishly.
"It kind of goes on and on," she admits. "I would say it's easier for me to step back sometimes and say, 'Enough already, we can talk about this another time', but I'm equally as invested. So our children do hear a lot about work."