Jack O'Connell and Alfie Allen on Meeting the Real SAS for 'Rogue Heroes'

When the cast of EPIX's Rogue Heroes joined the wartime drama, they were able to meet members of the real Special Air Service (SAS), an experience that left a lasting impression on them all, Jack O'Connell and Alfie Allen told Newsweek.

The show examines the origin story of the British Army regiment, which was created by young Scottish soldier David Stirling in 1941 at the height of World War II, when the Allied nations feared they might lose the war to Nazi Germany and the Axis Powers.

Stirling (played by Connor Swindells) founded the SAS alongside fellow soldiers Robert Blair "Paddy" Mayne (O'Connell) and John "Jock" Lewes (Allen), and their mission was to go behind enemy lines and destroy their aircrafts and other weaponry.

Jack O'Connell and Alfie Allen on Meeting the Real SAS for 'Rogue Heroes'

When the cast met a member of the SAS before they started filming the drama, they knew he was "tough," O'Connell said. The actor shared how interesting it was to learn things about the elite military unit that had not been revealed to the public before.

"There was one fellow early on and we did a couple of prep weeks, that we had well before any cameras were around," O'Connell said.

"I think his name was John. [You] could tell that he was tough just to look at him, and we were all sat down in a classroom format because it was [about] stuff that we were doing and he was brought in, and gave us a sort of talk.

"He just kind of made us aware that a lot of the stories we're telling... we were getting access to stuff that has been kept out of the public domain," O'Connell added.

"I think that that was a shared view within the detachment: a lot of this stuff is classified but they prefer not to have novels or biographies made on a lot of this stuff, such was the preciousness of it to them.

"So he told us all of this and, under no uncertain terms, told us at the end to not eff it up," the actor added. "That definitely put a rocket up the bum (buttocks)."

Allen then joked: "You don't want those SAS guys knocking on your door when it's their story," sharing that the cast felt a great responsibility to tell their history as best they could.

Show creator Steven Knight, who also created Peaky Blinders, previously told Newsweek how he had the opportunity to meet the last surviving member of the original SAS, Mike Sadler, now 102, whose stories elevated the drama to new heights.

Tom Shankland, who directed the drama, also shared how SAS members worked with them during rehearsals to help inform the actors' characterization of their heroes.

On Their Characters and Filming 'B*****' Raid Scenes

Both O'Connell and Allen were keen to portray real people onscreen, with the latter also relying on the letters his real-life counterpart Jock Lewes had written to his sweetheart Miriam "Mirren" Barford during World War II.

"To be able to get the opportunity to play a real person as an actor is really something I always enjoy, just because you get kind of real source material to work from," Allen said. "So, for Jock, there was a book of published love letters called Joy Street between him and his partner, Miriam Barford. That was super helpful."

Allen added that he enjoyed how Lewes, Stirling and Mayne were "completely different" but were "brought together" by their desire to do something unheard of, adding that the show felt important to tell now.

"That's something that definitely I thoroughly enjoyed, but also playing men that were fighting to stop the spread of fascism. I think that's especially appropriate now," Allen said.

Rogue Heroes saw the cast recreate key moments from the early days of the SAS and their aforementioned fight against fascism. One such scene sees the group conduct raids on several enemy air bases where they destroyed planes and launched attacks against the soldiers stationed there.

O'Connell said of filming these moments: "You just feel a bit b*****, I suppose, with a loaded Tommy gun, albeit blank rounds, and a cool bit of dialogue.

"Obviously you want to do it justice, to the reality of it, but when you've got like these planes on set, and you're recreating this airfield, and everyone's in the uniform, etc. it's quite easy to imagine the reality of it [...] it is intimidating. It's a headspace that is fascinating to imagine."

Allen said that filming in the Sahara Desert in Africa was "amazing" but also "tough at times," which meant there "were highs and there were lows" on set.

"We started shooting when it just started to get quite hot, and then it just got hotter," the Game of Thrones actor said. "So, the locals were literally looking at us like we were mad to do this and, so, true to form in terms of the characters we were playing.

"It was kind of a massive course and kind of forced method-acting in terms of the environment, the Sahara Desert being there, just having trouble with the equipment and the machinery we were using so, in the traditions, it was tough," Allen said.

"So we got a tiny, tiny taste of what it was like to operate there. It was impossible at times [but] it was great."

Rogue Heroes airs Sundays at 9 p.m. EST (6 p.m. PST) on EPIX.