The idea of Ethan Hawke and the low-budget “The Purge” winning the weekend box office would have been unthinkable weeks ago.

But as Friday’s release date approached, “The Purge” surged. By the weekend, it was handily the box office king with a $36.4 million estimated haul. It mauled “The Internship,” starring Vince Vaughn and Owen Wilson, and finally knocked “Fast & Furious 6” off its No. 1 position.

“As Friday came around and the numbers started coming on, the movie went beyond what anyone ever thought was at all possible,” says “Purge” producer Jason Blum.

So how did this $3 million film succeed? Some key factors:

Great concept: “The Purge” might not have wooed the critics who skewered it, but it was hard to find fault with the intriguing premise – a peaceful future where the government has sanctioned that crime be legal for a horrifying 12-hour night once a year.

It allowed for the imagination to run wild with trailer scenes involving masked figures breaking into an affluent home.

“Every once in a while a movie comes around that has a premise that just captures people. “The Purge” is one of these movies,” says Dave Karger, chief correspondent for the movie ticket site Fandango.com. “The concept is so different. It makes people want to see the movie.”

Blum says he has seen this fascination at screenings across the country. The concept of government-allowed violence gets people passionate. “People want to talk about it. They need to share their thoughts,” says Blum. “It really struck a nerve.”

Horror sells: Blum has made his name in the micro-budget horror film business producing hits such as the “Paranormal Activity” franchise, 2012’s “Sinister” (with Hawke) and 2011’s “Insidious.”

“The surge on ‘The Purge’ is a testament to the fact that horror films are among the most consistent performers at the box office,” says Paul Dergarabedian, box office analyst for Hollywood.com. “‘The Purge’ is providing some much-needed horror-type relief from the typical summer fare. And it’s a profit machine.”

Ethan Hawke: Blum is good friends with the art house actor and convinced him to do his first horror film, “Sinister.” Now “The Purge” brings Hawke’s biggest opening weekend ever. The “Before Midnight” star has a significant fan following and is ideal casting as a caring father trying to protect his family.

“He’s a really strong, really relatable actor and he really helped sell this concept,” says Blum. “Ethan makes it believable.”

Social media: After the first “Purge” trailer dropped in April, “The Purge” took off on social media sites.

“There was so much conversation on all social media platforms,” says Doug Neil, senior vice president of digital marketing for Universal Pictures, the film’s distributor. “YouTube, Tumblr, Facebook all amplified the conversation.”

Universal stoked these flames with a clever campaign on these platforms which included a faux public service radio announcement on music sites such as Spotify and Pandora.

Twitter too was a huge force with topics #WhatWouldYouDo and #SuriveTheNight trending. On Thursday, the topic #IfThePurgeWasHappening took off on Twitter. “People were just jumping into the conversation,” says Neil.

This intense activity gave Blum the clear sign the movie was going to take off, even if social media does not show up on pre-release tracking estimates.

“It shows that social media is changing the movie business,” says Blum. “There was a ton of online talk about this movie.”

The micro-budget: Some might find the $3 million budget limiting. But the small financial risk allows filmmakers to take chances on a project in the hopes of getting big rewards. And the money people are willing to cut the checks.

“If you pitched this concept to a studio and said you want a $20 million budget to make it, they would rightly say, ‘You’re out of your mind,’“ says Blum. “It was a risky proposition. What allowed us to do it was that we didn’t spend a lot of money.”

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