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 rather than just seeing if he could even figure out that the guy was lying.
Why was the guy lying? What was he covering up? I didn't share that information with the player. That's for them to discover and/or deal with.