The Player is the person controlling Stanley's movements throughout the duration of The Stanley Parable (with the exception of the Not Stanley Ending). The Player explains Stanley's diversity of thought, movement and actions.
The Player's existence is called into question during the Not Stanley Ending; the Narrator realizes that a Player is deciding what Stanley does, and not Stanley himself. He then leads Stanley/the Player back to the Two Doors Room to be confronted with the decision they made that started the whole charade.
No matter which door the Player takes, the game will always have problems: the right door will cause the entire game to distort and succumbed to chaos due to "Narrative Contradiction," while the left door will lead the Player to the boss' office, now equipped with a voice receiver that prevents the Player from proceeding any further due to the game's system not supporting speech recognition with microphones, in addition to parodying the trope of Stanley being a silent protagonist.
The game appears to have its own comprehension about what a real person/the Player is: in the instructional video shown in the Not Stanley Ending, a real person is a human whose choices always make sense. If in addition, you are talking to a person that doesn't make sense, then that person should not be considered "real."
If the Player gets the Narrative Contradiction to occur, the Narrator will regard the Player as "a child." However, if the Player advances to the altered Boss' Office, the Narrator becomes angry at the Player for not having "the kind of respect Stanley shows for his choices."
- If Stanley stays in the Broom Closet long enough, the Narrator will assume that the player has died, and will alert someone to take their place.
- The Player also appears as the protagonist in The Stanley Parable Demonstration, where the Narrator gives them a tour around the facility before seeing the demo. If the Player waits in the waiting room, or plays the Eight game, the Narrator will start getting annoyed.
- In the Not Stanley Ending, the game can't work because Stanley is not being controlled by the Player. That could also explain the intro video stating "You will play as Stanley. You will not play as Stanley."