The film's director, speaking with a fan on his Formspring account, added that Thanos will indeed speak in the film, which suggests they'll have to cast the villain's voice actor (and find someone who's as intimidating as Vin Diesel, who voices Groot).
"We've already said that Thanos plays a part in it and is a character in it. That in and of itself should tell you it's connected to the other worlds," Feige said, adding that "Ronan The Accuser is very much the main bad guy. And Thanos is lurking above it all. You will learn more about Thanos in Guardians, for sure. Certainly you'll get more than the one turn around and smirk. You'll get much more than that!"
Feige also added some more details, including the fact that Gamora starts the movie as a villain before joining up with the Guardians.
"They are an unlikely band. It's pure circumstance that brings them all together throughout the course of the movie," Feige told Empire. "Rocket has been genetically and mechanically altered, he's an experiment from one part of the galaxy. Groot is a species that happens to look like trees look, and is quite communicative if you can understand the different inflections in the way he says, 'I am Groot.' Peter Quill left Earth in the mid-'80s, but when we meet back up with him he's very much a citizen of the universe. Drax has one goal and one goal only, which is to kill Ronan because of a past tragedy. When we meet Gamora, she's a bad guy working with Ronan and Nebula and Korath, and by extension for Thanos. Gamora doesn't necessarily like doing that anymore, and tries to find a new path for herself. But she has a reputation as the deadliest woman in the galaxy that people aren't greeting her with open arms."