NPC
Purpose of NPCsThese characters are available for drop-in players or during the trial period if a new player wants to join the group.
Reasons to start play as a NPC
Amount of work: It takes a lot of work for the dungeon master and the player to create a character and make sure he/she fits into the storyline and world background. It's best to save this work for when a player is going to be a long term regular player.
Continuity: Having new characters appear for one or two sessions, then suddenly disappear stretches the believability of the campaign. As the campaign progresses, these characters are often fairly high level for the surrounding area. If such characters are so common that they just appear and disappear at seeming random, then why are the heroes important?
NPC's becoming PC's (and vica versa)
NPC to PC: A player (new or old) is welcome to take a NPC and convert it into their character. This works great for the campaign since the character is already known in the campaign. The player may change any part of the character that doesn't involve race (ethnicity), gender, and outward appearance.
PC to NPC: If a player ends up leaving the game, their character PC becomes a NPC if still alive.