I created a quest alias with fill type "unique actor", selected my NPC and attached a script to it:

scriptname MyAliasScript extends ReferenceAliasevent onInit()	Debug.trace("alias init")		  ;prints: alias init	Debug.trace("alias "+GetActorReference()) ;prints: alias NoneendEventevent onActivate(ObjectReference akActionRef)	debug.trace("activate")  ; The event is never dispatchedendEvent

Apparently the alias isn't filled automatically with the selected NPC when the quest is started.
How do i fill it?

Did you remember to assign your NPC a "Persist Location"?

What is a Persist Location? I placed it somewhere by draging it in the cell render window.

http://www.creationkit.com/Alias#Fill_Type:
http://www.creationkit.com/Object_Reference#Persist_Location

I set a persist location, but its still not working. Even if I use "specific reference" fill type instead of "unique actor".

Are you sure that the actor you're using is not already filling the alias of some other quest, one that "reserves" him/her? If that's not it, do you have any other aliases in your quest, which are possibly not filling and causing the quest to stop, therefore clearing the aliases?

Yes that was it! I had another reference alias that wasn't filled.

Thank you very much.