Reynolds — seen in “X-Men Origins: Wolverine” and “The Proposal” — plays a US civilian truck driver on assignment in Iraq who awakes inside a dark, confined space after his convoy is attacked.

It was written by Chris Sparling and directed by Spain’s Rodrigo Cortes, the filmmaker behind “The Contestant.”

“I’ll never complain ever again on a film set as long as I live,” he said at its screening at the Toronto film festival, saying filming was “emotionally and physically exhausting” and “pretty miserable.”

“It was tough. It was hard,” he said, commenting on the “spectrum of human emotions” felt by the character during the 90-minute film.

Shooting the first scene, Reynolds also lost his voice. “It created a strange kind of quality with my voice that I had to duplicate in the 16 subsequent days.

“So I’d wake up in the morning and put a towel over my face and scream until I was red and ripped my vocal chords apart,” he said.

There was a brief moment of levity on the last day, he said, when paramedics were invited onto the set in case something went horribly wrong for Reynolds as the box was filled with sand, potentially suffocating him.

“I thought: ‘That’s interesting, there are paramedics on the set today, what are we shooting?’” he recalled.

Because Reynolds said his lines in a “fevered pitch” the crew kept overacting and ripping the lid off the box, fearing for his safety, forcing them to shoot it over and over again. “It was funny,” he said dryly.

Screenwriter Chris Sparling said he originally set out to write a script for a very low budget movie and ended up with 79 pages about a man trapped in a box.

Seventeen days of shooting would actually turn out to cost more than two million dollars. Seven boxes were constructed for different camera angles.

“This is the kind of film that shouldn’t be done. It was nonsense and foolish,” commented Cortes.

But he went ahead, he said, indeed because “it was impossible to make a movie like that. I decided to go against common sense.”