"Before I shoot someone, I do extensive research," Klein told ET on Wednesday. "I was drawn to more of [Bieber's] snapshots and paparazzi pictures, so my approach was to do more of a private shoot, which I call a private collection."
Using a Polaroid camera from the '70s, Klein aimed to capture raw and real -- aka no re-touching -- images of the 21-year-old.
"There was no editing by art directors," he said, noting that the shoot was "simple, spontaneous and really documenting him in the way he looked [that day]. He came in looking pretty much like that, with that haircut, so there was no manipulation at all."
"It was kind of trying to make this idea of an intimate look at Justin, instead of this fabricated pop icon's idea of photographing a celebrity or a pop star," he added.
That includes detailed images of marks on Bieber's back from a recent "cupping" session.
"He just came from his acupuncturist, he had a neck injury," Klein recalled. "At one point, the makeup artist was going to take them down or remove them with makeup, and I just said, 'Leave them, because I want to reveal the true imperfections and the reality of what he really looked like.'"
The old-school method of capturing and developing each individual photo kept Bieber intrigued, Klein said, and the singer even brought a few ideas of his own to the shoot.
"He wanted to take a picture like he was a photographer, taking a picture of a girl," Klein recalled of one specific shot. "I like working with people that are creative and have an artistic integrity."
And while Klein aimed to "remove all the artifice of what people think about [Bieber]," it was the Biebs' idea to pose in the buff.
"We had probably 20 racks of clothes, and he actually looked at an overview of the clothes and he didn't even really want to shoot the fashion," Klein said. "Everything was more like flesh and black and simple."
But he did make an exception for a few eerie props: a terrifying clown mask with flame-red hair and a lone knife.
"I've used a lot of guns as props, so I thought I'd move on to knives," Klein said with a laugh.
For more on Bieber's Interview feature (conducted by Martha Stewart, believe it or not), watch the video below.