(FIXED)
After playing around with it for some time, I've solved my own problem.
The script identifies the player's character with: $game_actors[x]
The x represents the number of the character/actor in the database. I.e. 001, 002, 003, etc.
The script identifies party order as: $game_party.actors[x]
Party leader is the character in the top position. Top position is numbered as 0, not 1.
So, $game_party.actors[0] refers to the character in the top position.
Use a Conditional Branch by going into the fourth tab, click Script at the bottom and copy and paste the following:
$game_actors[x] == $game_party.actors[0]
Of course, replace x with the character's database ID. This will check if that specific character is in the top position. If they are, the game will carry out whatever commands you put under it. Leave Else blank.
You will have to make multiple Conditional Branches for each character you want to include.