When to use "whom" and "who" when the direct object is also doing an action

"I just saw that guy throw a ball."

"[T]hat guy," the direct object, is now doing the action of "throw[ing]." So, could one ask, "Whom did you see throw the ball?" or should it be "Who did you see throw the ball?"

I would naturally replace "that guy" with "him" if I wished to say it that way, so--assuming that is correct--does that mean "that guy" is always treated as the object despite doing an action?

Sorry if my phrasing is confusing; thank you for any responses.

Edit: This has been marked as a duplicate, but the linked question does not answer mine.

Solution 1:

For the sake of illustration, let’s simplify your example and use a pronoun:

I saw him.

You can see that him is an object — the receiver of the verb see. That’s why we use the objective pronoun him rather than the subjective he.

Here’s the question for the answer:

Whom did you see?

Note that the verb see still needs an object — a receiver of the verb. So we use the objective pronoun whom rather than the subjective who.

Linguist Bryan Garner notes that the demise of whom has long been prophesied, but that whom is not dead yet. He continues:

In any event, writers in the 21st century ought to understand how the words who and whom are correctly used.
_{Source: Garner’s Modern American Usage}

If you want to “talk English” (as linguist John Lawler says in the comments above), go ahead and use who. You’ll likely sound abnormal otherwise. But if you want to write English, I suggest — at least for the time being — that you seek the correct form.