die Contenance wahren — To keep one’s composure. Duden defines Contenance as composure in a difficult situation.
jemanden aus der Contenance bringen — To make someone lose their composure.
einen kühlen Kopf bewahren — To keep a cool head. Duden: not to become nervous and to keep the overview.
den Kopf verlieren — To lose one’s head. Duden: to lose the overview, one’s calm, one’s composure.
Wer schreit, hat Unrecht. — Whoever shouts is in the wrong.