What is to act as

To act as

Act \Act\, v. i.

To exert power; to produce an effect; as, the stomach acts upon food.
To perform actions; to fulfill functions; to put forth energy; to move, as opposed to remaining at rest; to carry into effect a determination of the will.

He hangs between, in doubt to act or rest.
--Pope.
To behave or conduct, as in morals, private duties, or public offices; to bear or deport one's self; as, we know not why he has acted so.
To perform on the stage; to represent a character.

To show the world how Garrick did not act.
--Cowper.

To act as or To act for, to do the work of; to serve as.

To act on, to regulate one's conduct according to.

To act up to, to equal in action; to fulfill in practice; as, he has acted up to his engagement or his advantages.

to act up, to misbehave

I find the Moon method more gently empowering for crystals that are primarily intended to act as guardians.

What Lurie evidently had in mind was for him to act as a sort of public advocate, arguing their case before the courts of law and of human opinion.

He said in a whisper that we had to act as if nothing was out of the ordinary.

Lepidus grew important only when someone was required to act as go-between where the two major partners were concerned.

What is "to act as"