There's also a difference between not worrying as much about the divide between player and character knowledge (which is what I was talking about) and telling them everything (something you've now introduced to the discussion).
What I'm trying to describe is to not make everything a puzzle. I mean, if you have a mystery or a puzzle-box type scenario, that's one thing... being careful with what information is shared makes more sense. But not every scenario is meant to be a mystery.
Like, "who killed the duke"... sure, that's a mystery. What does my character perceive? That's not a mystery. I find that there's not always a distinction between these two types of situations, and I think there should be.
For instance, in a recent game, I told a player that an NPC was lying to him. I didn't call for a roll or anything else... I just said "you know this guy is lying". I did this because that's what seemed to make sense, and because it was much more interesting to put the character in that position and see what the player would do.