A smile can represent a lot of things, happiness, comfort, a way to greet. However, the inherent trust of body language can be broken quiet easily with a fake smile. Whenever I see fake smiles from people it is always a bit of shock and I'm less likely to trust what this person is saying.
Which is why I'm quiet surprised by how well animators, artists, whoever, are able to convey such a convincing, for lack of a word, fake smile on their characters.
As an example we have Rita from Sakurasou no Pet na Kanojo. When she first arrives into the dormitory of Sakura, I had that initial suspicion of her, because something was off about her face and I couldn't quiet tell why; later I figure out it was that psuedo-smile she attempt around most of the people in Sakura. Kind of weird to base anything on such a vain thing, but a person trying to deceive people in any sense should be presented with some doubt.
The next example was Megumi from Suki-tte Ii na yo. Megumi from the start gave off the impression of "I'm going to impress you with all this fancy stuff, so like me", and that didn't even have to be reveled by her body language. Later on her tactics failed her and she ends up being rejected by most of the people around her. That is when Megumi showed her very last fake smile and broke down to her insecure self that she's been masking for sometime.
It's quiet an interesting thing to look at deception and what people want others to think of them, but all in all it is nothing but a lie that some people can pick up on quiet easily. So essentially, when you have a slight bit of suspicion toward a character, just keep your eyes open and just observe how they act; it is quite often that this character will be the provoker of the protagonist, to shake up the plot.