Actor, n.
Etymology: [L. actor, fr. agere to act.]
1. One who acts, or takes part in any affair; a doer.
2. A theatrical performer; a stageplayer. After a well graced actor leaves the stage. Shak.
3. (Law) (a) An advocate or proctor in civil courts or causes. Jacobs. (b) One who institutes a suit; plaintiff or complainant.
Shades of meaning
Fernald has no entry of its own for actor; he lists it under agent, cause.
Where he writes about it:
But in common usage, especially in business, an agent is not the prime actor, but only an instrument or factor, acting under orders or instructions. — from agent