From Time's long article about the production:
They did that by working with IMAX to solve three problems. The camera is deafening—Damon has likened it to trying to act with a blender next to your face—so they invented a casing around the camera called the blimp to muffle the noise during intimate, dialogue-heavy scenes. The camera’s short magazines require frequent reloading, so they developed a protocol in which the entire set remained silent so the actors could, in van Hoytema’s words, “stay in the zone.” And the camera had grown so large that actors couldn’t see around it. “Which is, of course, terrible for an actor that you cannot play against the person you’re supposed to see,” says van Hoytema. Their solution: a set of mirrors that allowed the actors to make eye contact.