Cast and Crew of 'Rogue One' Open Up About 1st 'Star Wars Story'

— -- Sometimes, potential “Star Wars” leaks come from the most unexpected sources.

A scare for Diego Luna came one day on the set of “Rogue One: A Star Wars Story.” In between takes, Luna, who plays rebel Capt. Cassian Andor, was wearing his costume and using Facetime to talk with his young son, Jerónimo, when he heard a “click.”

“And then I see the screen going white. He took a picture of my costume! And I go, ‘No!’ I made his mom go erase it,” Luna says. “We couldn't share anything, you know? My son took like six months just to get the name of my character out of me.”

Secrets come with the “Star Wars” territory. But at the recent “Star Wars Celebration” fan event in London, the cast and filmmakers behind “Rogue One” sat down with ABC News to offer a few tantalizing hints.

They say “Rogue One” will be a grittier, darker film than “Star Wars” fans may be used to. Think of it as really putting the “war” into “Star Wars.”

“Even if the stories tend to get somewhat edgier, slightly darker, they maintain the values of ‘Star Wars,’” Lucasfilm President Kathleen Kennedy said. (Lucasfilm is a division of ABC News’ parent company, Disney.)

Do all the main characters make it out alive? Maybe not, Kennedy says.

“The characters in what is truly an action adventure story are going to run into some fairly complex obstacles. So they will confront things that may result in a little bit of sadness, but a lot of hope,” Kennedy told ABC News. “There's always hope.”

Coming to theaters in December, “Rogue One” is being billed as the first stand-alone “Star Wars” movie, taking place outside the core nine-episode film saga. In the “Star Wars” chronology, “Rogue One” falls between episodes three and four.

The plot involves a group of rebel spies led by Jyn Erso (Felicity Jones) to steal the secret plans to the Death Star. (Those stolen plans are what Luke Skywalker and friends eventually use to destroy the Death Star in the original 1977 movie, now called “Star Wars Episode IV: A New Hope.”)

“She's incredibly determined,” Jones says of her character. “She's an absolute survivor and she is thrown into a situation that she isn't expecting in any way and it's a situation that demands great courage from her. At the same time, she has moments of great vulnerability and is not perfect.”

At the “Star Wars Celebration” fans were also treated to a brief glimpse of Darth Vader, returning to the big screen for the first time since “Episode III: The Revenge of the Sith” in 2005.

Vader’s presence on the “Rogue One” set even intimidated the director, Gareth Edwards.

“I was just reduced to being a kid and I looked at the rest of the crew and everyone was just looking like they were four years old. Going up to direct him was like, ‘Hi there, hi, my name's Gareth,’” Edwards says. “You suddenly realize, when you do a shot you look on the monitor and you get goosebumps again.”

Riz Ahmed, who plays rebel pilot Bodhi Rook, jokes that Vader is a buzzkill.

“He's a very passive-aggressive man. I'm not afraid to say that,” Ahmed says. “He would kill the vibe on set.”

“Rogue One” also stars Oscar-winner Forest Whitaker, Mads Mikkelsen, and Ben Mendelsohn. Donnie Yen and Jiang Wen, new to Western audiences, are both huge stars in Asian cinema.

“Every day was a challenging day for me and I'm glad,” Yen says. “I will become part of this history.”

Alan Tudyk provides a motion-capture performance for a new, computer generated droid named K-2SO, who he says could beat C-3PO in a fight.

“I'm seven-foot-one, so my arm, my reach is huge,” Tudyk says. “He's not even going to get near me! C’mon!”

At a time that when lack of diversity in Hollywood is getting more attention, the “Rogue One” cast is proud the film reflects modern society.

“It is important in this day in age, especially when you're dealing with conflict, to try to deal with some of the questions that might exist,” Whitaker explains. “What happens when you have small portions of people who feel disenfranchised and can't have their own world?”

Most of the “Rogue One” secrets remain intact, for now. But the cast did offer a few final hints. Well, sort of.

“I can tell you this: You will see me with people you know already,” says Mendelsohn, who plays an imperial bad guy named Director Orson Krennic. “I've never told anyone what I just told you so there's that.”

Ahmed adds, “I would say that the name Bodhi itself has some significance. You like that?”

“Whoa,” Tudyk responds. “That's great. I want to know more now. K-2SO is into macramé. You're not going to find that anywhere but here. It's an exclusive.”

“I want to tell you everything, except I really don't,” Mendelsohn jokes.

ABC News’ Carolyn Durand and Ashley Riegle contributed to this report.