The original record or minutes of a transaction, negotiation, examination…a formal or official statement of a proceeding. In extended use: the accepted or established code of behaviour in any group, organization, or situation.
Environmentalists want Clinton..to go further by encouraging the immediate negotiation of a protocol that would commit industrialized nations to a 25 percent reduction of greenhouse gases..by 2005. Mother Jones, January, 1973.
To be professional means to follow protocol. The actor’s protocol is set partially by the union agreement, movie-making precedent and set etiquette. When the protocol is broken professionalism is difficult to maintain.