I am currently running Princes of the Apocalypse for a small group, with a few of my own embellishments and alterations to characters.
Most pertinent to this question, Captain Merosska, the leader of the Feathergale Society freely admits that there are air cult activities in the area, and that they're a problem. However, he claims that the air cult is an insidious group infiltrating the Society that he is working on purging, while he instructs the party to find and destroy the earth cult, who happen to be the air's enemy. He gives a small speech about how the Feathergale Society is his family, and he is dedicated to remove this corruption himself and wants no outside interference. In short, his goal is to get the party to leave him alone and fight his enemies.
All the while, the players were not fully trusting of this. They passed their Insight checks by a wide margin (we're talking 6 Deception vs 23 Insight), so I had to reveal what they learned. The problem I have is, just saying
"he's lying, he's a cultist and is playing you"
is cheap, and ends an entire mini arc before it can begin. As a rule, I aim less for "you know they're lying/telling the truth" with Insight and more towards "they're clearly shifty/nervous/watching a particular party member/etc." to clue people in without giving any certainties or mind-reading powers. So, I gave them this:
He is clearly suspicious of you, and is nervous keeping you in his home. He wants you to leave as quickly as possible, though he is being diplomatic about it. He does seem genuine when he describes the society as his family, though.
I thought that was enough, but resulting player chatter made me realize that the players interpreted this as him being generally honest but cautious, and worthy of the party's trust. I don't necessarily have a problem with this, but I feel like I might have cheaped out on my players and steered them wrong after they got such fantastic Insight checks, which are meant to be checks that read the honesty and intention of a person.
Did I give too little information to my players? How much information are players entitled to with successful insight checks? Can this be reconciled with not giving away entire dramatic reveals at the wrong moment?
Maybe I'm just too attached to my deception arc. This whole situation feels reminiscent of the Goblin Dice issue to me.