But my whole point is that if I make any kind of knowledge gaining type check, whether that's insight, history, nature, etc, I'm going to give them all the relevant information on a success. If they're investigating a door for a trap they're going to also see other things that an investigation check could reveal.
Maybe. But like I said I'm pretty generous. That said, if the extra info about the door is relevant at all, I'm probably gong to give the players that info regardless.
If I make a perception check looking for rats in the cellar, I'm not going to tell them they miss the clown in the corner because it's not a rat.
Sure, that's fair and I'm not saying that's the case. Unless it's a ninja clown.
The goal of a perception check is always the same: do I see anything hidden or unusual. The goal of an insight check is always simple: gain more knowledge about the target NPC. I don't need anything more than that.
Sure, but I'll give that to them regardless, but the players might not need to roll any dice to get it, thus why I want to have them tell me what they're doing. Kind of like I want to know if they're interested in learning about something before deciding what information they're going to get. I might info dump them and not need to.
Telling me specifically what they're looking for or trying to see is superfluous because I'm not going to play "gotcha" DMing. A successful insight check will always reveal the same info no matter what they were initially looking for.
Neither am I if you think that's what is going on. But I also operate on the basis that the player has a goal in mind and I want to know what it is so I can adjudicate the result of their proposed actions.
And just to clarify, how much information would you provide? Your previous example were all things that are noticeable about a person that some kind of check would provide in addition to the truthfulness of a response. Or rather they are things that indicate such truthfulness, or are observable traits without making checks. Your dismissive to the gnome example I would just provide, probably as part of an introductory description because its a character trait I want the players to be aware of for that NPC.
Now, I find that the reason the players want to use this kind of check proactively falls within one of two very broad categories: 1) they think the DM is out to get them; or, 2) they already suspect something because the DM has lead them to think so through previous interactions/scenes/whatever.
I generally want to use something like Insight as a way to get the player to be able to interact with an NPC in a way that they can't otherwise. They can't see the NPC fidget and sweat when questioned about a specific topic. But a successful check will tell the players Ned is uncomfortable and probably lying because he's getting physically uncomfortable. The players can't see the NPC react to talking, even a visual medium like a video game doesn't do it well, despite
L.A. Noire and
Elder Scrolls IV: Oblivion's best try. So I'm by default going to provide visual information about an NPC reactions if the players want to use a check to find something out. I still want to know what they want to find out specifically, since it will change how much information they get about that thing.
As an example of the visual medium. My wife and I were watching one of those shows about game wardens in the US. They officers are questioning a guy about hunting. She noticed that he kept taking layers of clothing off the more they asked him about his behavior. That's an observable behavior I can use to describe a successful check with Insight. I'm not necessarily going to info dump that he keeps avoiding eye contact with Hamlet unless it adds to the scene or gives more information the players don't already have.