causal agent vs doer

causal agent and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
causal agent	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hyponym of causal agent; that is, doer is a word with a more specific, narrower meaning than causal agent:

causal agent: any entity that produces an effect or is responsible for events or results
doer: a person who acts and gets things done

causal agent (noun)	doer (noun)
any entity that produces an effect or is responsible for events or results	a person who acts and gets things done