It’s difficult to imagine Ridley Scott’s sci-fi/horror classic “Alien” without the clear-minded, strong presence of Tom Skerritt as Dallas, the captain of the ill-fated Nostromo.

But originally, the actor turned down “Alien,” which celebrates its 40th anniversary on May 25, though he thought Dan O’Bannon’s script read well. “There was nobody involved at the time apparently,” said Skerritt. “I read it and thought, ‘it’s solid. It’s not a great script but it’s solid enough I can see it. But it was a $2 million budget! I thought, okay at 2 million bucks this might be an Ed Wood movie.”

As fate would have it, he went to see 1977’s “The Duelists,” for which Ridley Scott unanimously received the award for best first work at the Cannes Film Festival. “I was just blown over by ‘The Duelists,’” noted Skerritt.

“I thought, this is a masterpiece. It’s a painting. I thought I wanted to remember who this this guy is. Then I got a call from one of the producers of ‘Alien,’ Gordon Carroll, and he said “They’ve kicked up the budget and a guy named Ridley Scott is doing it. I said, ‘I’m sold.’ All I needed to know was Ridley was going to do this and he would make magic out of it.”

Scott wasn’t the first choice of the producers, which also include David Giler and Walter Hill, to make magic out of O’Bannon’s script — Ronald Shusett co-wrote the original story with O’Bannon.

“I was the fourth choice,” noted Scott, adding that Robert Altman had been offered the movie before him. “You don’t offer Bob ‘Alien,” he explained. “It’s not his thing, you know?”

Someone, he said, recommended the producers see “The Duelists.” Scott acknowledged he didn’t know how they connected a 19th century period drama to a sci-fi thriller about an alien creature running amok and killing off the members of a spaceship.

“They sent me a script and I read it,” said Scott. “I loved it. I was in Hollywood within 32 hours. Once I was there, they said ‘Look, the budget is hovering just under $4 million.’ I did ‘The Duelists” for $850,000, so the figures sounded right to me. I said, ‘Well, what I will do first is go back [to London], look at this carefully to see if there’s anything I need to adjust.”’

It took Scott three weeks to storyboard the film. “I was drawing all day. Then I flew back to L.A. with the boards, and they suddenly saw they had a different kind of movie. The boards told them that. So, that’s the power of being able to visualize. It went up to $8.2, I think. Then we went to $8.6 by the time we finished.”

Besides Skerritt, the cast includes Sigourney Weaver in her featured role debut as Ripley, the warrant officer who goes mano-a-monster with the alien; Veronica Cartwright, as the emotional navigator Lambert; the iconic Harry Dean Stanton as the engineering tech Brett; John Hurt as executive officer Kane; Ian Holm as Ash, the science officer who actually is an android; and Yaphet Kotto as chief engineer Parker.

“Alien” made $105 million worldwide back in 1979 — the adjusted gross is $283.5 million — and spawned three sequels, two crossovers with the “Predator” franchise, and two prequels, 2012’s “Prometheus” and 2017’s “Alien: Covenant,” both directed by Scott. A third prequel, which he will direct, is in the script phase.

The tagline for “Alien” was “In space, no one can hear you scream.” Ironically, audiences couldn’t stop screaming. As soon as the baby alien lifeform bursts from Kane’s stomach in a shower of gore, “Alien” is a non-stop thrill ride.

“Nobody had ever done anything like that,” offered Cartwright.

Scott also followed the rule that the best horror and sci-fi films adhere to: Never reveal too much.

“You don’t show the monster too many times because you’ll get used to him and you never want to get used to him — ever. That’s always been my thesis. The best screening room in the world is the space between your ears, which is your brain. So, it’s learning to tap into the human brain to show just so much. Let the brain do a lot of the work. That’s where you start to tap into people’s anxieties.”

Michael Seymour’s Oscar-nominated production design added to the anxiety. “The Nostromo was one giant set on a stage at [Shepperton], all built together so that they could do those kinds of shots,” said screenwriter Scott Essman. “Where you’re going from room to room and you’re going down hallways and it’s underlit. It’s very suspenseful and you don’t know where the creature is from one moment to the next.”

Essman noted that “Alien” was one of the creepiest films ever made in terms of its distinctive look. “The cinematography, the lighting and mood and how Scott moves the camera around the ship is really haunting.”

“Additionally, H.R. Giger’s creature designs were deeply disturbing, decidedly erotic, and wholly unique in a genre film,” said Essman.

“Everything was very claustrophobic,” added Cartwright. “We had tons of smoke. It was so filthy.”

Skerritt recalled the set was so dark, the actors couldn’t really see the walls. “It created a sense of floating out there in the part of the universe that doesn’t have a lot of light. That gets to you. The whole atmosphere that he built around the set itself was quite effective for all of us.”

“Alien” was produced before CGI, so all of the creature effects were done practically on the set, including the terrifying, bloody “birth” of the alien baby.

Hurt, said Skerritt, was “laying down on a platform beneath the table and the t-shirt he’s got on has been sliced enough so that it will be pushing up through that t-shirt.”

While Hurt and the effects were being set up, the cast were in their dressing room for hours waiting.

“They had filled the chest with offal [and fake blood] and we retched as we walked in,” said Cartwright. “The whole place was wrapped in plastic. Everybody was in raincoats. We were like okay, what’s happening? They said I would get a little blood on me; little did I know I would be leaning directly into a blood jet. I got blasted in the face and then that ‘Oh god,’ came out. Those were all our first reactions in literally one take.”

“Alien” went on to win the Oscar for best visual effects.

Speaking of Hurt, actor Jon Finch (“Frenzy,” “Macbeth”) was originally cast as the doomed Kane, but an illness on set caused him to drop out, says Scott.

“I always operate on my films, so there’s only one camera,” said Scott. “So, I’m on camera, doing my first shot, first take, and I notice Jon has gone yellow. So, I walk over to him and he said, ‘I feel terrible. I’m a diabetic.’ I said, we better get you out of here. You need some insulin.”

They got him into an ambulance “and I never saw him again,” Scott said. He would eventually cast Finch in his 2005 epic, “Kingdom of Heaven.”

Later, Scott went to see Hurt, who had been in strong contention for the role. Over drinks at Hurt’s cottage, Scott offered him the job. “We had him in the office the next day, got him measured. I was shooting by just shortly after lunch.”

Cartwright say Scott worked in a “really interesting way. He hardly ever talked to us. He was a workaholic. He could go for hours and hours and not sleep. He had such a vision of what it as going to be like.”

As to why “Alien” quickly entered the pop culture landscape and still endures four decades later, Scott said, “I think ‘Alien’ captured our most primordial fears. It’s particularly special because it’s not gilded with any characterization other than what you see is what you get — minute by minute with these people. That’s really why a lot of people were scared to death. It’s because they are living in it, minute by minute, and eventually, second by second.”