Shrouded in mystery, 10 Cloverfield Lane has become one of the most buzzed about films of 2016.
Since its trailer first debuted online, fans were surprised and excited to learn there was another Cloverfield film -- this one being a "blood relative" to the hit 2008 sci-fi film about an alien monster attacking New York City.
The new movie, about a woman who wakes up in an underground bunker with two strangers to learn there's been a worldwide chemical attack, was so mysterious its cast -- John Goodman, Mary Elizabeth Winstead, and John Gallagher, Jr. -- didn't even realize that both films were connected until the very last minute.
"We all just follow [producer J.J. Abrams'] lead and we know that he's got a master plan in mind when it comes to how to unveil these movies," Winstead tells ET. The actress herself didn't even know about the film's connection until hours before the trailer was released online. "And I think he likes to keep the experience pure for the audience, so that they come in and go on a real rollercoaster thrill ride."
Part of that thrill is picking up on all the Easter eggs that Abrams has become notorious for planting in his projects, with one of them being a surprise cameo by Bradley Cooper. In a blink-and-you miss moment, the actor can be heard on the phone with Winstead in the opening scene of the film.
Abrams fans will of course remember that the Oscar-nominated actor once starred in the filmmaker's ABC spy drama, Alias, alongside Jennifer Garner.
"We just remained friends," Abrams tells ET about working with Cooper. "When we were working on the movie, I wanted you know to make sure we had a voice that we loved. We were just talking about who would be that great voice and so I asked, 'What if it was Bradley?'"
"I texted Bradley and I asked him if he was interested in doing this," Abrams explains, "and he as the sweetheart he is texted back and said, 'You know whatever you want.' He's just the greatest, so it was a very easy thing to do."
The cameo itself was even a mystery to Winstead, who revealed that she filmed the scene over a year ago. "Now we get to say, 'Bradley Cooper is starring in our movie,'" she says. "It's pretty cool."
10 Cloverfield Lane hits theaters on Friday, March 11.