WANTED: Someone to go back in time with me.

This is not a joke. You'll get paid after we get back. Must bring your own weapons. Safety not guaranteed. I have only done this once before. (Backwoods Home magazine, 1997.)

From small ads in local magazines do big phenomena grow. The ad above, which appeared in a California publication more than 20 years ago, first caused ripples locally, then a rumble on television, and eventually a full-blown internet frenzy. Now, courtesy of director Colin Trevorrow and writer Derek Connolly, it's the launch pad for a new science fiction comedy drama out next week.

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"For whatever reason, it really grabbed people's imagination," says Trevorrow.

At the centre of Safety Not Guaranteed, a winner at Sundance, is a magazine intern, Darius, who has been told to find out more about the ad's author. The part was written for Aubrey Plaza, who stars in the American television comedy Parks and Recreation. Still relatively unknown here, Plaza is to women under 25 what Tina Fey of 30 Rock is to an older generation – smart, savvy and cool.

"She really represents young women of a certain age in a lot of ways in America right now," says Trevorrow.

The director had sent the script to the production company of Mark and Jay Duplass, the writer-director brothers who helped pioneer the "mumblecore" style of filmmaking (low budget, low lighting, naturalistic dialogue) in such films as The Puffy Chair before going on to direct more mainstream comedies, including Cyrus with John C Reilly and Marisa Tomei.

"They came on board initially just as producers to help us get the film made. They're really good at that, they know how to get things done cheaply," says Treverrow. The budget was $750,000, of which about $10,000 was the art budget. "Extraordinarily low," he explains.

Big names were initially sought to play the ad's author. In the end, Trevorrow looked closer to base. "I thought it was just cooler to have Mark play the role." Having Duplass play a supermarket shelf-stacker also added to the film's all-important "Is this for real?" factor.

Audiences had to wonder, says Trevorrow, if this guy was crazy or not, and whether he was capable of building a time machine.

"I'm not so sure if it had been a big movie star who accomplishes spectacular things all the time if we would have doubted for a second that he really is doing what he says he can do."

In the film, Darius needs all her sang-froid to survive being an intern. It's a life of coffee-making and no wages with which many young people can identify these days, says Trevorrow. "The internship seems to be your only option coming out of college."

Back in his day – he's a relatively ancient 36-year-old now – things were different: "My internship was far more enjoyable than hers."

It helped that he was an intern on the comedy show Saturday Night Live. He worked in the talent department, looking after the guest stars.

"It was a pretty cushy internship compared to what I could have been doing. Amazingly, I got to be inside the process and see a lot of stuff a lot of other interns probably weren't able to see. Part of that was my boldness in just sort of pretending that I belonged places that I probably didn't belong. I would just go and stand and watch sketches during a live production. People just thought I was one of the writers."

It was while looking after Quentin Tarantino that the young Trevorrow was given the most daunting task of his SNL internship. "He gave me a huge responsibility to go to his hotel room and get his notebook." Trevorrow was amazed. "Why would you let someone ride in a car with your notebook for 20 minutes? I tried to honour his trust."

Just like the small ad that led to his film, Trevorrow was at the centre of an internet tempest recently. He gave an interview at a film festival, hinting that he was about to start work on a movie that would be a big talking point. Six months later it was reported that there would be a new Star Wars movie. Internet rumour-mongers put two and two together and ...

"It got as crazy as only the internet can get," laughs Trevorrow, who lives in Vermont with his wife and their two children (a son aged four and a daughter born this summer). The film he had actually been asked to direct was a remake of The Flight of the Navigator, the 1986 drama about a boy who becomes lost in space. Though adored by many, it's not quite Star Wars: Episode 7.

His next film will be Intelligent Life, another science fiction indie set in the real-life United Nations Office for Outer Space Affairs. Then it will be time to direct Navigator. At the moment things are still at the draft script point.

"What it's really about is convincing ourselves that there needs to be a new version of this story, that we really can build something that will both appeal to my son and their generation, but will also satisfy a lot of people who really do care about that movie. I'm one of them."

Safety Not Guaranteed opens on Boxing Day at Cineworld Renfrew Street; Cineworld Edinburgh; Cameo, Edinburgh; Glasgow Film Theatre, January 28-30.