Is it possible to make NPCs either appear or disappear based on conditions like a quest's stage?

Say at a certain point in a quest I want an NPC to walk away and disappear. At that same time, a different NPC will appear elsewhere. How would I go about doing that? And if it's a script, could someone walk me through that?

Thank you very much.

Sorry I'm not very good with setting up quests etc. but take a look at the gray fox and the thieves guild quests -- they have him do that a few times and it should give you some ideas on how to do it.

The simplest way is using two commands:

.Disable <-- this Disables the NPC
.Enable <-- The Enables the NPC

I'm not much good with packages, so don't know enough to explain how to do it by making them walk away, but something like this might do it :

  If ( GetStage [QuestID] >= 20 )     If ( Player.GetInCell [NPC's Cell ID] == 0 )        [NPC's ID].Disable     EndIf  EndIf

Very rough, but something like that can at least cause them to disappear, even if it's sometimes a bit odd when it's done in practice.