"The Incident Commander [IC] is the only position in the Incident Command System that is always filled regardless of the size or complexity of an incident. The IC has the responsibility for the overall management of the incident. Whatever functions or responsibilities that are not delegated to others remain the responsibility of the IC. The engine Company Officer who responds to a call of food on the stove will fill the IC's position regardless of the local agencies' routine rank title for that officer. The initial IC's first responsibility is to assess the situation (size up) to determine the problems, issues or concerns that the crew is confronting."
Coffee Break Training - Command and Control Series No. CC-2013-1
United States Fire Administration: http://www.usfa.fema.gov/