"I actually made a real friend for my life, you know?" Gyllenhaal adds.
In the movie, an international space crew discovers life on Mars. However, their exuberance over the discovery quickly turns to horror when their research methods have unintended consequences. Reynolds and Gyllenhaal play scientists on the mission.
"We sort of breathe life into it, to use sort of a Frankenstein kind of comparison," Reynolds explains.
"It was terrifying," Gyllenhaal says. "But there was something really interesting about how terrifying it was."
While Reynolds' character is a risk-taker, Gyllenhaal is the cautious veteran on the team. Despite their characters not agreeing on everything, in real life, the actors hold a great deal of respect for one another.
"[Reynolds] has so many strengths as an actor that I don't have -- his sense of humor being one of them and his comic timing, which is pretty unmatched," Gyllenhaal says.
Likewise, Reynolds had glowing things to say about his co-star as well.