The person who knows how becomes calmer when everyone else discovers why it matters.

When a situation becomes urgent, everyone else is discovering the stakes and learning the problem simultaneously. Someone who already knew how is only dealing with the first half of that, which is why they seem calm. It is not temperament, it is having done the learning earlier.

Study the thing that would matter most in a crisis before there is one. Preparation done in a quiet month is what produces composure in a loud one.