Furthermore, some emphasis is put on volitionality in Sowa's system: the initiatior of an action can be either Agent or Effector, depending on whether (s)he causes the action voluntarily or not -something that is often difficult to decide even when a context of a sentence is given.
the Agent role is extended in VerbNet to 'internally controlled subjects such as forces and machines', but it is easy to confuse this role with Theme.