A calm situation is any part of gameplay where the timing of actions and events is not absolutely critical. That is, while the attitudes of the characters may not be calm (for example, a heated argument), the urgency of the situation does not demand a rigid turn structure.
During a calm situation, all characters are free to take any actions they wish, and at any time. They simply announce what they are doing - if a check is required, it must still be passed. The narrator will also control any non-player characters at this time and in the same fashion.
Should any action by any character cause the situation to turn critical or deadly, the characters have entered a dire situation.