The player role

The player is a role, not a block. No entity is the player by construction: an author writes characters, marks the ones eligible to be played playable, and hands the role to one of them in the story's start block.

create Alex
  a person
  playable
  starts in the Greenhouse

  Mud on your boots, dirt under your nails.

before the game starts
  change the player to Alex
end before

create the player is removed (parse.removed-create-player). There is no header field for it either — assigning the role is an action, and a field would give the language two spellings for one thing.

playable

A bare composition word marking a character eligible for the role. It is person-only: playable on anything else is analysis.playable-non-person. It says nothing about who is playing now — a playable character the story never assigns is simply an NPC.

Everything else about the character is written the way it always was. starts in stays on the character, because where someone begins is a fact about them, not about the role.

before the game starts

One block per story, at the top level, running once after the world is built and before turn one — ahead of the first room description, so what it changes is what the player first sees.

before the game starts
  change the player to Alex
  move the brass key to the Potting Bench
end before

Its body takes effect statements only. phrase and emit there are analysis.start-block-narration, which points at the header's prologue: — the block runs before any turn exists to carry prose. Opening prose belongs in the header.

The role must end up filled. A story with no start block is analysis.start-block-missing; one whose block never assigns the role is analysis.start-block-no-role; a second block is analysis.duplicate-start-block. The runtime never reaches turn one with nobody in the role.

change the player to <character>

The same statement anywhere, not only in the start block — it is what moves the role during play.

create the standing mirror
  in the Long Gallery

  A tall mirror, silvered and slightly warped.

  after the player touching
    change the player to Viola
    phrase the-swap
  end after

The target must be a person and must be playable (analysis.player-target-not-person, analysis.player-target-not-playable), both checked at compile time.

The character who held the role keeps their own clauses and goes on acting as an NPC. This is the distinction clause heads draw: the player in a head means whoever holds the role when the clause fires, while a head naming a character means that character whether or not they are being played. See on and after for how a head chooses between the two.