What to call the executor of an action?

I can't find the right word to describe the object that performs an activity/action.

• Doer
• Executor
• Performer
• Actor

The term is to be used in an application where you may choose an "object" (from a list), and then retrieve its activities.

So what should I call the "object"?

Context: The "object" may be anyone or anything which is to do certain activities. Think of a worklog where you'd like to log activities that has been done. Given the object type, you'd might have a different set of activities to log.

Object (1. line support member) Activities { Create ticket, Assign ticket, Notify customer, Close ticket}

Object (Purchasing employee) Activities { Receive order, Confirm Order, Ship Order}

One option, which shares its etymology with action, is agent. Both come fundamentally from the Latin agere meaning to do.