Yes the order of execution is indeed, OnExecute once and only after that, OnUpdate until the action is finished by an EndAction call.
Many included actions also do use OnExecute for caching some data and OnUpdate to make use of them, just like you are trying to do.
Maybe there is something else going wrong here? Is this custom action relevant to Mecanim the only one you have issues with, or there are others as well?
Gavalakis
