The gamemaster prepares the game session for the players and the characters they play (known as player characters or PCs), describes the events taking place and decides on the results of players' actions. The gamemaster also keeps track of non-player characters (NPCs) and random encounters, as well as of the general state of the game world. The game session (or "adventure") could be described as a play, in which the players are the lead actors, and the GM brings the stage, the scenery, the basic plot on which the improvisational (unplanned) script is built, as well as all the bit parts and supporting characters. Gamemasters can also be in charge of RPG board games, making the events and setting up challenges.