I am fairly new to D&D; however, I have years of roleplay experience. Last night I attempted to charm a shopkeeper, and the DM didn't tell me if I succeeded or failed — he seems to do this with everything. Anyway, having read the description of charm person and then reading several views on it by others, it seems the common consensus, if the NPC passes the save and is not charmed, is that nothing should happen. However, what happened is this:
The DM told me the shopkeeper's eyes started to glow slightly and to tell her what to do and so I asked her to give me some potions. I understood it as though if her eyes changed then yes, she was charmed. Apparently this was not the case and she pulled a crossbow out on me instead, so I left the shop. I really think I've been cheated here, but I want your opinions as I'm unsure whether I need to talk to the DM or not.
Like I said, the common consensus I found is that if an NPC is not charmed then nothing at all should happen until the spell ends, when it says they know they are charmed, but they have to have been charmed in the first place to know this.
Best Answer
Only your DM knows
Why the shopkeeper's eyes started to glow slightly? We don't know. Maybe the place itself is magical, it might be a part of the mystery the DM prepared for you. Or the eyes weren't really glowing, since it was just a figure of speech. Only your DM knows this for sure, so you should ask her (him) first.
Why the shopkeeper reacted so aggressively? We don't know either. We can guess why, but only your DM knows this for sure. There is a couple of reasons tho, why the old lady could do that.
1. The spellcasting act itself is obvious
Unless your character is a sorcerer using subtle spell metamagic, the Charm Person spell requires verbal and somatic component, that means "chanting of mystic words" and "forceful gesticulation or an intricate set of gestures" according to the book. So an unknown person comes to your shop, casts a spell (it failed on you but still) and then requires to give something for free. Looks like an obvious robbery attempt for me.
2. The target of the spell knows it was charmed
According to the spell description, the creature knows it was charmed "when the spell ends". The DM could rationalize, that it does not matter if the target succeeded or failed the saving throw — the target will know regardless of the reason why the spell ends.
You should talk with your DM if this bothers you
D&D is a game you can't play "wrong" as long as all participants are having fun. If some things bother you, you should talk about it with the DM. Be specific, name things you feel unfair or frustrating. Focus on your feelings, not on being "right" or "wrong" according to the rules. A decent DM will keep this in mind and adapt the game.
You can consider talking to your DM not by telling them what they did poorly, but focusing on how you felt. e.g. "Hey, last session, when X happened, I felt confused/frustrated because I couldn't figure out Y." You accurately know what you felt, so you can speak confidently about that. Your DM is unlikely to get defensive when you don't directly criticize them, so is more likely to help (thanks @BBeast for the summary).