THEPACT, an inventive haunted house mystery lately arrived from the Sundance Film Festival, is the debut feature from writer-director Nicholas McCarthy. The film stars Death Valley’s Caity Lotz as a tough girl – tattoos, dirt bike, scowl – who returns to her family home after her mother’s death and her sister’s mysterious disappearance. It has been hailed as this year’s Paranormal Activity, although unlike that feature, McCarthy, a fortysomething Sundance veteran, is not exactly an overnight sensation.

“It’s a good story,” laughs the filmmaker. “But it doesn’t include the years and years of making short films and just trying to get a break. There’s no clear path to getting a movie made. It’s a strange, fickle industry. The Pact happened so quickly. But only after a decade of writing screenplays and meeting everyone I could.”

Since the late-1990s, McCarthy has enjoyed a series of breaks that ultimately fizzled out. Working as part of a Godardinspired collective known as Alpha 60, his work initially came to the attention of Sundance programmer Mike Plante more than a decade ago. Cry for Help, his short about meeting zombie Jesus in the afterworld, premiered in Utah in 2005; Chinese Box, a conversational miniature, debuted there in 2009; The Pact, the embryonic version of his first feature, played in 2011.

“Sundance has been incredibly important for this movie,” says McCarthy. “I’m not just saying that because they like me. I think the programmers there do a great job of finding interesting and challenging films, even if sometimes those films don’t get written about or get overshadowed by Paris Hilton coming to town.”

Four years ago, McCarthy married TV commercials producer Alexandra Lisee. The couple have a young daughter, Agatha. The short version of The Pact ought to have been the last throw of the dice before straight-world responsibilities kicked in. But then Jamie Carmichael, the president of Content Media’s film division, happened to be sitting in Sundance’s Egyptian theatre.

“It’s funny,” says McCarthy. “I had this bottle of champagne that I kept in the fridge for years and years. My wife wanted me to open it because she felt it had attained some kind of mystical significance and hold over me. That was the bottle I was going to open whenever I finally got to make a feature film. But when?

“There were so many different green lights along the way. There was the first meeting when Jamie Carmichael said ‘I want to make this movie’. There was the day I turned in the script and got positive responses. There was the day the lawyers got involved. There was casting. There was shooting. And even shooting, you never know. Films have shut down during production before.”

He finally cracked open the bottle three weeks after the shoot: “By then,” he says, “It was only an afterthought.”

On first inspection, The Pact is just another post-found footage wow. McCarthy’s feature does not pose as a video diary or webcam chronicle. It is, however, recognisably part of the new dirty digital democracy, the sort of films we were told would be mandatory in the aftermath of The Blair Witch Project.

“I think digital has finally fulfilled the promise of being its own medium,” says McCarthy. “Certainly in the indie sector, it has become something that can work alongside film rather than competing head on. It has its own look and language.”

So what is so special about The Pact? Why has it transcended the bubbling primordial sea of low-budget horror flicks?

For one thing, it’s beautifully performed. A properly gothic picture that wrings plenty of eerie possibilities from an uncanny Freudian home, the film’s generous quota of spook-house scares are delivered with precision between McCarthy and remarkable Final Girl Lotz.

“We came across Caity during the casting process,” says McCarthy. “We had a casting director in LA and we saw a lot of young women for the part. And honestly, Caity was right from the moment she came through the door. There’s a kind of toughness and gravity to her movement. I think it has something to do with the fact that she has a background in dance and martial arts and parkour.”

The house, too, becomes a primary character in the supernatural drama. But behind the scenes, the track dwelling was an even livelier presence.

“I met a guy whose mother had recently passed away who said we could shoot at her house,” explains McCarthy. “So all the things you see in the house are hers. She had lived in this suburban house in San Pedro for decades and the place was loaded with depressing seventies wallpaper and a lot of Catholic iconography. When I walked around the place, I realised it was the kind of environment that I hadn’t really seen in a horror movie before outside of seventies pictures.”

McCarthy, though raised Catholic in a first-generation Irish-American family, and a huge fan of Spanish Catholic gothic such as The Blood Splattered Bride, swears he was a sceptic until the lights started flickering during the original shoot.

“There were a lot of little things,” says the film-maker. “Lights went on and off independently. Files were corrupted and none of us could figure out what happened. And finally, after the shoot, I walked through the set on my own to check we hadn’t left anything behind. And there was a box, sitting on the dining room table, that none of us had seen before. Inside was a collection of the previous owner’s things and instructions for her funeral arrangements.”

Impressed, he returned to the same house to shoot the feature length version and kept many of the original props.

“We figured that presence has been lucky for us. We wanted to make sure she was still watching over us.”