Address the characters, not the players

Ask the characters what they do. Tell the characters what they see and hear. Tell the characters what's obvious to them. Don't say, "Brian, what is Vahid doing to keep awake?" Say instead: "Vahid, what are you doing to keep awake?" Don't tell Jamie, "Rhianna can tell that he's lying." Say (in Iwan's whiny voice) "No way, Rhianna, you know I'd never do that," and then (in your own voice), "But he looks away and you can just tell he's full of crap."

By speaking this way, you keep the game focused on the fiction. You push the players to think, speak, and respond as their characters. You establish fiction as the characters perceive it, because that's who you're addressing.