Terrence Malick spent 40 days filming his latest drama “Song to Song” throughout Austin, in his trademark style, without a completed script. The days were long — starting in the morning, with only a 30-minute break for lunch. The actors were even shot in the car, moving from one location to the next, just in case it turned into something.

“With new cameras, you can quickly accumulate a lot of footage,” Malick said at a rare public Q&A at SXSW on a rainy Saturday morning. “We had an eight-hour first cut. We thought, ‘Is this a mini-series?’ It really could have been. It took a long time to cut it down to a manageable length.”

Malick, who lives in Austin, started making the movie in 2012. But he finally unveiled the experimental drama as the opening night feature at this year’s SXSW Film Festival on Friday night. Although the reclusive director didn’t sit through the showing, he emerged the next morning with his lead actor Michael Fassbender, in a conversation moderated by Richard Linklater.

Malick shed some light — in his own way — on his unconventional process. “We keep rolling to keep it spontaneous,” he said of his style. He likes to portray “bits and pieces” of his characters’ lives, as opposed to traditional narratives.

“You never know at the end of the day what you actually got,” Malick said. “The editing takes a longer time than usual. You have to ask the patience of the studio or the financier. Sometimes more than once.”

The original title for “Song to Song” — about a music executive (Fassbender) as he drifts through a series of tortured relationships — was “Weightless,” based on a Virginia Woolf quote. The most specific direction Malick gave Fassbender was to channel Satan in John Milton’s “Paradise Lost.” At times, he’d yell between takes, telling him not to look like he’s posing.

The rest of the cast, which includes Ryan Gosling, Natalie Portman, Rooney Mara and Cate Blanchett, bob in and out. There are cameos from Patti Smith and a blink-and-you’ll-miss-it Val Kilmer as a rock star. “We shot so much more than we finally used,” Malick said. “We’re very sorry not to use more of his stuff.” (“I wanted to see more of the Val scenes,” Fassbender said, remorsefully.)

Malick also revealed why he doesn’t work with storyboards. “If you try to make things happen, they start to feel presented,” he said. “The action has been premeditated. It starts to feel like theater, which is wonderful in its own right. But you don’t want the movies to be like theater.”

Malick got in the film business in the ’70s with indie cult classics “Badlands” and “Days of Heaven,” but before that, he spent a year as a lecturer at MIT. “I quickly realized I wasn’t capable,” he said. “I didn’t understand what I was meant to be teaching. I felt like I had to do something else.”

Fassbender said he was happy to finally appear in a Malick film, after a previous collaboration they had talked about didn’t come together. The best perk was not needing to memorize lines, which usually takes him a long time. “It’s very liberating when you’re not carrying dialogue,” Fassbender said. “You’re actually in the moment.”

But there was a downside to working with a director who improvised so much. “I’ll be acting my socks off over there,” Fassbender said. “The next thing, I look over, and Terry if filming a beetle.”

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