This code system http://terminology.hl7.org/CodeSystem/med-admin-perform-function defines the following codes:

Code	Display	Definition
performer	Performer	A person, non-person living subject, organization or device that who actually and principally carries out the action
verifier	Verifier	A person who verifies the correctness and appropriateness of the service (plan, order, event, etc.) and hence takes on accountability.
witness	Witness	A person witnessing the action happening without doing anything. A witness is not necessarily aware, much less approves of anything stated in the service event. Example for a witness is students watching an operation or an advanced directive witness.