Changing the Party Leader

In the default scripts in RPG Maker XP, the actor located at index 0 of $game_party.actors is considered the "leader" of the party. Therefore, the idea here is to move the actor to index 0 of the party. Here's how you can change the leader:

def makeLeader(actorID) # Makes the actor with ID _actorID_ the leader
 added=false
 for i in 0...$game_party.actors.size
  if $game_party.actors[i].id==actorID
   actor=$game_party.actors.delete_at(i)
   $game_party.actors.unshift(actor)
   added=true
  end
 end
 if !added
  # Add actor to party
  $game_party.actors.unshift($game_actors[actorID])
  # Maximum 4 party members
  while $game_party.actors.length>4
   $game_party.actors.delete_at(4)
  end
 end
 # Refresh player
 $game_player.refresh
end

The example below changes the first actor to actor 2 (which, by default, is Basil):

makeLeader(2)