executant

A person who executes a dance or piece of music; a performer.

[…] no composer or dramatic poet has ever pretended to be able to perform all the parts he writes for the singers, actors and players who are his executants.

A person who puts something into action, such as a plan or design.

Also of importance are the other theatricians - the costume designer and/or executant, the lighting designer and/or executant, the technical director, […]

After all, Cave speaks about orders being so manifestly illegal that the person carrying them out must or ought to have been aware of their unlawfulness. From this alone, one can draw the conclusion that, if the executant of the order actually knows of its unlawfulness, he ought to be convicted, no matter whether the order is or, as in John Doe's case, is not manifestly illegal.