The calmest one at the audition is usually the one who rehearsed in an empty garage for weeks.

Calm under pressure looks like temperament and is mostly inventory. Someone who seems unbothered has been through the material so many times that the room cannot take it from them. What you are watching is not the absence of nerves. It is nerves with nothing left to destabilise, which is the quietest advantage available to anyone.

Rehearse the thing you are dreading in conditions closer to the real one, out loud, more times than feels necessary. Learn the opening by heart. Calm on the day is almost always a memory of the rehearsals.