Pressure true story: Who was James Stagg and how accurate is the film?

New World War II drama Pressure recently opened in UK cinemas, on the back of some very positive reviews from critics.

An adaptation of David Haig's 2014 play of the same name, the film stars Andrew Scott as RAF meteorologist James Stagg, who had a key but often unheralded role in the run-up to D-Day.

Scott is joined in the cast by Brendan Fraser as Dwight D "Ike" Eisenhower, with key roles also for Kerry Condon and Damian Lewis.

And speaking to Radio Times ahead of release, Haig explained why he'd initially decided to write about Stagg and his crucial intervention in the war.

"I came across [the story] because the Royal Lyceum Theatre in Edinburgh wanted a story about an unknown Scottish hero," Haig revealed when asked about the origins of the project.

"The theatre director John Dove and I researched and discovered James Stagg, and what a subject. He fascinated me because he was an unsung hero in the fringes of history who affected our future profoundly."

So who was James Stagg? And how accurate is the film? Read on for everything you need to know about the true story behind Pressure.

James Stagg was a Scottish meteorologist who worked closely with Eisenhower to plan the D-Day invasion of Nazi-occupied France.

Stagg was instrumental in predicting terrible thunderstorms, which led to a 24-hour delay in the D-Day landing plans.

Though he may not have been the easiest person to get along with, Stagg played an absolutely vital role and his reservedness contrasted with Eisenhower's personality.

"Having started to research him [we] found he was a man of quite quiet restraint, could be terse, brusque, potentially superficially unlikeable," Haig explained.

"[So it was interesting] to balance him with the explosive energy and weight of Eisenhower.

"And to have the triangular influence of Kay Summersby (Eisenhower's personal secretary, played by Kerry Condon), who had developed an interdependence with Ike, using that interdependence to influence and communicate with Stagg on the other side of the triangle.

"It became a fascinating thing and made it a play and film about people as much as it is about a quirky aspect of history."

When researching the part, Haig went to Stagg's surviving relatives. "[Stagg's family] really were a process of enforcement, enforcing or reassuring me that I wasn't far off target in my interpretation of the father," he said.

"And they sort of reflected him to a degree in the sense that they are quite self-contained men, but have a sort of strength within them, and so that was very interesting meeting them both."

Lewis's character – the legendary British war hero Bernard 'Monty' Montgomery – clashes with Stagg during the course of the film, feeling threatened by the seeming lack of military experience from the latter.

"Monty was a particular flavour, and he wasn't liked by everybody," Lewis explained. "He was adored by his men. That's not the Monty we saw in this. We didn't have time. It's not a film about Monty. He's there to, I think, give a sincere and seriously committed military argument to come up against Stagg's meteorological argument, so that's the purpose he serves.

"He condescends to Stagg. Manages to remain polite, just, but he's clearly irritated by, why are we having the liberation of Europe dictated to us... the timing of it dictated to us by a weatherman?"

He continued: "Monty was well known for meticulous preparation, and he'd been planning it for months with the other guys and thousands of men already had been sleeping on the boats and eating on the boats. You couldn't leave them there longer. It was bad for morale, bad for health, bad for security. Germans might have discovered what we were up to."

How accurate is Pressure?

Pressure is largely accurate in its retelling of events, although as is often the case in a Hollywood film about historical events, it does take a fair few liberties with the truth.

The relationship between Stagg and Esienhower – as well as his disagreement with American meteorologist Colonel Irving P. Krick (Chris Messina) – are based in fact, and it really is true that the Scot played a huge role in delaying the D-Day invasions and essentially preventing the mission from becoming a disaster.

However, the film does somewhat simplify these events. In truth, there were many more meteorologists – working across three different teams from the United States Strategic Air Forces in Europe, the Meteorological Office and the Royal Navy Meteorological Service – and there naturally wasn't room to include all of these in the film.

Still many of the other details seen in the film – including the fact that the Associated Press mistakenly announced the invasion in advance, that inflatable tanks were used by Allies as decoys and that a report from Blacksod Point weather station in Ireland ended up being absolutely vital – are very much true.

It's also correct that the training exercise known as Exercise Tiger, which was in many ways a dress rehearsal for D-Day, was a huge disaster, with that fact shown to weigh heavily on Eisenhower in the film.

In short then, while there are a couple of simplifications and some of the arguments might have been dressed up a little for dramatic effect, Pressure is very much a thoroughly researched film based largely in real history.

Pressure is now showing in UK cinemas.

Check out more of our Film coverage or visit our TV Guide and Streaming Guide to find out what's on. For more TV recommendations and reviews, listen to The Radio Times Podcast.