The man gave me a slow bow and ambled away, revealing a/the woman who'd been standing behind him.
Should it be "a" or "the" and why?
Both sound kind of okay for me.
"revealing a woman who'd" returns 1 Google result. Using "the" returns none.
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You will hear both. "A woman" emphasizes that her identity is irrelevant. "The woman" emphasizes that a specific woman was behind him. I doubt either is ungrammatical. Nevertheless, "the woman" is better because we are identifying a specific woman by her specific location at a specific time. If it is relevant to emphasize that the woman has not been identified by name, you can say "the woman, as yet unnamed, who had been standing behind him."
In general I'd say you use "a" when both parties aren't expected to know which specific thing you're talking about; this can be when the specific thing isn't important ("I ate an apple"), or when the thing is first introduced in the text/conversation ("As he turned the corner, he saw a woman. She was his long-lost sister!"); i.e. the person talking/writing has a specific person in mind, but the person they're talking to or the reader doesn't already know which it is.
When you use "the", there is an expectation that the person talking is thinking of a specific person, and that the person reading/listening can tell which one it is. This can be because the specific entity was introduced before ("I saw a car. The car turned around the corner"), or because something else in the sentence gives more information on which specific thing it is ("The car that turned around the corner was red"). In the earlier sentence "He saw a woman", if you wrote "He saw the woman", the reader would ask themselves "wait, the woman? Which woman? Did they talk about a woman earlier? Was there some indication there is only one woman in this universe?". The woman is specific, but the reader doesn't know about her yet so they can't know which specific woman she is, so you wouldn't use "the". Unless you wanted the reader to ask themselves such questions of course, which you might for stylistic reasons.
In this case both work grammatically, because "who'd been standing behind him" specifies things enough that you can get away with "the". (obviously only true if there is only one woman standing behind him!).
So both are grammatical, and probably both would work for what you want to do. It's likely one is better than the other but it's hard to tell without knowing more about the context, what's up with this woman, etc.
revealing a woman who'd been standing behind him
Could suggest surprise. As in, you are first introducing the concept of this woman; the person watching the scene wasn't expecting to see a woman there, and there one appears. We also don't know yet if this woman will be important or not.
revealing the woman who'd been standing behind him
Does more to suggest that the woman was there all along, and the person watching knew that, and even if they didn't, the narrator had her in mind when they started the sentence. There is also a suggestion that the woman is going to be important in the next few sentences at least.
Both put the reader's focus on the woman, but the second does it more like "now look at this woman" and the first is more "hey, there's a woman!". Either of those could lead to the woman being important or not. So you really could use either one, whatever you want to do. Even if one is better than the other in the context that you're writing, and a native English-speaker or seasoned writer would know which one it is, if you use the other one it will probably be fine and the difference hardly noticeable.
Both are perfectly fine. The difference is how they set up the context for later sentences or build on earlier ones.
Is the woman of particular importance? Is rheir other women in the wider passage in that we need to distinguish the one hiding behind him from others? Will she play more than a passing role in the events yet to occur?