Choose Your Champion

• WWizards of the Coast2010-06-15
  Official Ruling
  The player is chosen as the ability resolves. Once a player is chosen, it's too late for other players to respond by casting spells.
• WWizards of the Coast2010-06-15
  Official Ruling
  The targeted opponent may choose you. In that case, only you can cast spells and attack with creatures before your next turn begins.
• WWizards of the Coast2010-06-15
  Official Ruling
  The effect doesn't wear off until just before your next untap step (even if an effect will cause that untap step to be skipped).
• WWizards of the Coast2010-06-15
  Official Ruling
  Players other than you and the chosen player may still perform game actions besides casting spells and attacking with creatures: They may block with creatures, activate abilities, perform special actions, and so on.
• WWizards of the Coast2010-06-15
  Official Ruling
  The targeted opponent may choose themselves.

Rulings via Scryfall