One thing we know for sure about the upcoming mirror-based horror movie "Oculus" is that it's terrifying. But as MTV News found out when we sat down at SXSW with star Karen Gillan, writer/director Mike Flanagan and producer Trevor Macy is that like any good mirror, it hides its fair share of secrets.

You Can't Predict How Scary It Is

The day after the big premiere at SXSW, Gillan told us that the movie is so terrifying, even professionals can't handle watching the film.

"Someone told me there was a professional medium in the audience who couldn't watch. That's it! We've done it!" Gillan said, laughing.

It All Started With A Fear Of Mirrors

Originally shot as a short film in 2005, Flanagan used the inherent weirdness of mirrors to spin the story into a full-length movie.

"I've always found mirrors to be inherently creepy," Flanagan said. "We all have this image of ourselves, what we think we look like, and it's totally wrong. For one thing, it's backwards. And for another every mirror has its own imperfections and distortions built in that we don't think about. Mirrors freaked me out as a kid."

The Portable Overlook Hotel

Before the story expanded into being about two time periods, and two very different groups of people all entangled with the mirror, it was going to be an anthology.

"We looked at it like the mirror was this portable Overlook Hotel that you could pick up and put anywhere," Flanagan said. "So initially we wanted to do a whole anthology of shorts.

"Expanding it into a feature was tough, because it's a short about one guy alone in a room with some cameras. So everyone who saw the short and wanted to expand it into a feature said, 'Well there are cameras in the room, let's go found footage.' Which was not ever the way we wanted to go with the story. It took a number of years to find producers that were excited about doing something different."

"As that developed, it became a brother/sister thing, and that was the emotional core of the movie," Macy added. "That was the lens through which you could deliver these scares."

Doctor Who?

Gillan had a unique audition experience for the movie, one helped by her previous gig on "Doctor Who."

"Well, I'm sitting next to one of the world's largest 'Doctor Who' fans," Macy said, pointing to a bashful Flanagan. "So that was easy."

"I'm in my childhood bedroom in Scotland with my family," Gillan recalled, "and I get a call from my agent in America who's like, 'This director wants to speak to you from America!' And I was like, 'Woah! That's amazing!'

"We meet on Skype, I'm in my bedroom, it wasn't very professional. I was talking to him and all nervous, because he was this American guy, and he takes a swig of coffee from a cup, and it's a TARDIS. And I was like, 'Oh, we're fine, I've got this in the bag.' I just physically relax, and am like, 'All right, listen Mike.' "

"That was an accident," Flanagan said about the TARDIS mug, laughing.

The Mirror Has Two Faces

Filming with a reflective surface is difficult, but the crew managed to figure out a way around the challenge.

"That's one area where the short prepared us for that," Flanagan said, "because we learned really fast how difficult it was to shoot at a giant reflective surface without seeing yourself, and the crew.

"What we were able to do for the feature that we couldn't do for the short was have the glass or the mirror be on a gimbal, so we could adjust it just so slightly to get the cameras out, and get the crew out. Which also made for this really interesting effect. It's very subtle, so it's hard to notice, but it's not reflecting what it should be. It's always a little off, and I love that thematically."

"It's very unsettling, but you're not sure why, as you look at it," Macy added.

World's Longest Monologue

In the movie Gillan plays a woman trying to prove a haunted mirror has been causing murders for decades, including the deaths of several people very close to her. In order to prove the mirror's murderous intentions to her brother, Gillan goes through a nearly 13-minute monologue.

"We have a 12-and-a-half-minute scene where she's memorized one monologue for two days of shooting, and that takes a pretty unique actor," Macy noted.

"It's literally down to preparation," Gillan said on tackling the challenge. "It took me two months, or a month and a bit to learn that because it was a 13-page monologue of straight talking. And there was no emotional logic or conversation, it was just dates and facts. It was like a presentation.

"And the director told me at the beginning of the shoot that I have to be able to get through the entire thing in one take, and we would just do that over and over again so he could cut to a different angle on each sentence. That's the hardest thing I've ever done. I think I broke my memory."

The Light Bulb Moment

There's a moment glimpsed in the trailer where Gillan goes to grab an apple, but accidentally eats a light bulb instead. Gross.

"We had wanted to get in there this idea that what the characters are experiencing may or may not be real, and I have a natural phobia any damage to eyes, or fingernails, or the soft palette," Flanagan said, physically recoiling. "I can't deal with it. So naturally, it's like, let's do that."

"I read it and was like, 'that's cool,' " Gillan said. "But then we shot it and went, oh that's a trailer moment. Everyone looked at each other and were like, 'This is disgusting! Yes!' "

"Oculus" will hit theaters on April 11.