Even worse,

You create a complex NPC with a full plan, but the PC are in a murder hobbo mood and just kill them,
Then you have Bolin the Goblin, who suddently beg the PC for mercy, and someone wants to spare them, so now you have a whole session role-playing with Bolin the Goblin

I've learned that NPCs should have the bare minimum of background necessary for my sessions. If the players like the NPC, it they become useful later on, that's when I start fleshing them out. Of course, the NPC needs to survive long enough for that to happen.