I haven't read any of the books and have no idea what Littlefinger is like in them. But just out-and-out blabbing that he knows the single most dangerous secret in the Kingdom seemed out of character with what has been established about him in the show. It struck me as odd as soon as he said it. I mean, he seems like a guy who defines all the angles before judging which ones best suit him and then acting decisively. And in that situation, he had no angle to best serve him.
Maybe...maybe he';s operating on the principle that he's...I dunno... indispensable to the city and Cersi wouldn't dare murder him outright. Seems quite a gamble with a bitch who is bent enough to fuck her own brother and be complicit in the scheme to kill a king.
It was a fun scene, but it didn't really reveal anything about the two characters that we didn't already know. So, I question the decision to include it. Presumably, it didn't happen that way in the books? Or at all? Your post seems to imply that it was added for the show alone.
Yeah, the scene was added for the show. Which I don't mind but it's getting to be a bit of a habit with Littlefinger. For instance, the scene with the fisting prostitutes where he basically reveals his plan in season 1, which was also added for the show. I get that they have to reveal his backstory to the viewers but you could have another character (like Catelyn, say) divulge that he foolishly challenged Brandon Stark for her hand and got humiliated and nearly killed as a result. It's an important plot point that he learned
from that mistake and that's why he's a lot more dangerous now. It's almost the whole point of the character that he's deliberately adopted this persona, with the mockingbird sigil and the childish nickname. He's letting people underestimate him instead of making ostentatious moves for the sake of his vanity.
He's basically responsible for the entire War of Five Kings, remember. This is not a fool and he had nothing at all to gain from that encounter. Why even take the risk? Like Rockman was saying, the character in the books would have just let that slide and then set something up in the background. Actually, he's made it clear in the books that he considers Cersei to be an amateur in the backstabbing game and has said that he can basically just rely on her to destroy herself, so I doubt he'd even bother.
It was a good Cersei scene but I'm sure they could have found a way to do it (substituting another character for Littlefinger, for example) that wouldn't have required anyone to wear the idiot hat.