"Guardians is the harder one. With Belko, the premise is what people have to buy into," Gunn said Wednesday night at a launch party for the trailer. "With Guardians, people really go through every single little shot and try to figure out what the movie's about. And there's a lot of mysteries in Guardians 2."

Gunn said he spoke with Marvel's marketing team on Tuesday about how much to show from the film, which debuted a sneak peek in October.

"We were talking about how much to give away — and we're not going to give away very much," said Gunn.

A challenge with crafting a Guardians of the Galaxy trailer is that each shot can literally reveal new worlds (and new spoilers), wheras Belko — which takes place in an office building — has a lot more flexibility with which parts of the film it can show without spoiling anything.

"You can show all sorts of stuff happening and it doesn't really give away who lives, who dies, what the outcome is, what the mystery is," says Gunn of the Belko trailer. "With Guardians, it's such an emotional and physical journey, going to all these different places. It's like, 'God, do I really want to show this planet we go to in the middle of the movie? … Those are always hard choices to make."

Today, the first teaser for 2014's Guardians of the Galaxy is considered a classic. It established that it truly was something new for the Marvel Cinematic Universe, with everything from its use of "Hooked on a Feeling" to Peter Quill (Chris Pratt) being disappointed his arresters hadn't heard of Star-Lord, setting the tone for a marketing campaign that would help propel the film to unexpected heights.

But Gunn says that teaser almost never saw the light of day.

"That trailer tested not very well. We had another trailer that was very cheesy that sold the movie as something different than what it was — that tested a little better," says Gunn.

That cheesy trailer could have been released, but Gunn credits the marketing team at Marvel and Disney for going forward with the teaser fans ultimately saw.

"And at the end of the day, the [marketing] guys were like, 'This is what the movie is. We are selling what the movie is. Not something else,' " says Gunn. "You've got to sell the movie for what it is, and that's what they did. They really bought my trust with that on the first movie."

Here's the original Guardians of the Galaxy teaser from 2014:

The Belko Experiment, from director Greg McLean, will be released March 17, 2017. Guardians of the Galaxy Vol. 2 hits theaters May 5, 2017.