Dunkirk star Harry Styles had a '˜sing-song' on Gosport boat while filming war epic

The One Direction singer makes his acting debut in Christopher Nolan’s wartime epic Dunkirk – set to hit cinemas on Friday.

And now, Alan Watson – who is the captain of Second World War vessel HMS Medusa – has spoken out about his experience filming with the popstar.

The 61-year-old said: ‘Harry Styles wasn’t the least bit of what I expected him to be.

‘When I was told that he would be coming on board I thought that this will be some young person who has been thrown into fame and they will be difficult.

‘The first time I knew of him was a voice by my elbow that said “good morning, captain”.

‘I couldn’t wish for a more courteous, modest, well-behaved young gentleman.’

Mr Watson spent about three weeks filming pivotal scenes in the film last summer.

Numerous film takes were recorded by the Dark Knight director, often in challenging circumstances at sea, with two Imax cameras.

The 72ft boat never featured in the real Dunkirk rescue mission, which saw civilian and military ships of all shapes and sizes, scrambled to rescue the stranded 400,000 soldiers who had been overwhelmed by German forces.

However, the vessel was there during the D-Day invasion four years later in 1944.

Mr Watson added during filming he tried to make the famous popstar sea sick.

‘We were trying to throw the vessel around to make him sea sick but it didn’t work,’ he said.

‘He went up on to the bow of the ship having a sing-song. I was doing everything I could to cause him some discomfort but he was just singing a song.’

Medusa played a critical role at D-Day marking the entrance to a minefield which could only be cleared the night before by the minesweepers.

She spent 36 hours steadfastly keeping to her location so others could transit safely on their way to the Normandy beaches.

She was one of almost 500 similar vessels entirely built of wood, and powered by diesel engines.

They were not fast but had huge endurance travelling across the Atlantic in one go.

In her appearance in Dunkirk, she is seen transporting troops from port onto a destroyer, with explosions and gunfire blasting all around.

Alan, who will be appearing with the boat at Portsmouth’s Historic Dockyard on Sunday from 10am to 5pm, said it was a far cry from the vessels normal home in Haslar Marina, Gosport.

‘Everything felt so real, it was incredibly immersive – Christopher Nolan really did a stunning job of making us feel like we were really there,’ he said.

The film, which has received rave reviews, is out on Friday. Alan, of Chandlers Ford, also stars – albeit briefly.

He added: ‘It’s been a year since we did the filming so it’s absolutely fantastic to see it come to fruition. We have been holding our breath for a year. I can’t wait to see it.’