The Player Experience
Helper Information > Detailed Explanation of Themes & Concepts:
A player directly determines all actions their PC makes in response to the scenarios described by the GM. The exceptions are when they are under the influence of some external force beyond their control (hypnotism, confusion, etc.) – at which time they may become, in part or whole, a non-player character.
An NPC may be controlled by a player under the GM’s direction, who can alter the player's choice of actions for them based off their own unique knowledge of the character and scenario.
For more comprehensive information on the role of player, see the Wikipedia entry on players.
For more information and assistance for the role of Player, the
Player Services product may of interest.