grammar questions for grammarians only.. read the sentence?

All pronouns have a case, except for the pronouns This, That, These, Those, and Which. These do not change form.

There are three cases: possessive, nominative, and objective.

Because the pronoun we are discussing is not possessing anything, WHOSE can be ignored.

In your sentence, the best option is FOR WHOM ARE YOU ACTING?

This is the objective case, because it is the object of the preposition FOR. This is the best option because some grammarians dislike a preposition being at the end of a clause.

The second best is WHOM ARE YOU ACTING FOR?

This is more confusing because it looks like the pronoun WHOM is the subject of a linking verb ARE. If this were so, then it would be WHO. But it only looks like ARE is a linking verb because it is in question form, so the ARE precedes YOU instead of follows it. If it were written in declarative form, which is the best form to determine a pronoun's case, then it is clear that ARE is in fact a helping verb, not a linking verb, so WHOM is not nominative case.

The third option is incorrect; however, whom is slowly becoming obsolete, and thus this option is regularly used.

"For whom are you acting?" is the grammatically correct question.

Who is the subjective pronoun, whom is its objective form.

'You' is the subject, 'are acting' the verb (with participle), 'whom' is the object of the preposition 'for'.

That said, it is becoming more common to use who in place of whom (as in "Who does she think she's kidding?")

In your example, think of the Ernest Hemingway book For Whom the Bell Tolls.

Whom--

The pronoun in this case is recieving the action of the sentence, so you go with the "object" pronoun, whom.