Set party lead character - Events

First question: How do I change the lead party character?
As in, I've done the check to see if the character is in the party, but how can I set that character as lead of party?(shown on screen)

Second question: How do I go about taking a character from the current party, have them step out of the group, and do their own actions?
Do I have to use a seperate npc, then remove that pc from the party?

I can't remember an event that happens like this in either case (not saying its not done I just cant remember it) and the ones I thought did something along these lines look way to complicated for just setting the lead or making a character run off. Well, that does knida happen before sabin jumps off the water fall but I really dont remember any dialog there to find it either.