I want to script an actor to act as a follower but only during specific quests just like Delphine or Esbern during their respective quests.

I believe the key is scripting the PlayerFollowerFaction rank that's present on Delphine and similar NPCs and NOT the Current/PotentialFollowerFaction that regular followers such as Lydia have,
Changing this value during quest dialogue is the likey solution, unfortunately, my ignorance in the field of scripting has prevented me from investigating beyond this point.

Anyone know the solution?

Thanks