The 38-year-old directed and helped write the screenplay for the horror film, in which he also stars alongside his wife of eight years, Emily Blunt.
In the feature, a family are forced to live as quietly as possible in order to protect themselves from a clan of extraterrestrial creatures which hunt by sound. And as if John didn't have enough jobs to do, he also agreed to portray the monsters on screen.
"Nobody knows it because I've kept it quiet," Krasinski said when he appeared as a guest on "Jimmy Kimmel Live!" on Tuesday, October 16, before going on to explain that he wore a full-body motion capture suit in order to provide much of the physical performance.
His extra role came at the suggestion of his co-workers on set, who believed John would be the best person to help bring the characters to life.
"The amazing people at (Industrial Light and Magic, a visual effects company) asked, 'So how does the creature move?' " the film star recalled. "And I said, 'Well, this is how I think he's gonna crawl,' and they said, 'Well, why don't you throw on the suit?' And I was like, 'Totally!' "
"Then they took that picture and I thought I was auditioning for 'Lion King'," he added with a laugh.
The father-of-two also admitted that he was slightly apprehensive about working with his wife in a professional capacity. However, after the "Sicario" star insisted that she read the script while the couple were on a plane together, there was no going back.
"It was like a romantic comedy, where she was like proposing to me. She was like, 'You have to let me play this part.' And I think I just screamed 'yes' on this flight," John smiled.