What does "closed expression" on someone's face mean?

Question

Mr. Moony presents his compliments to professor Snape, and begs him to keep his abnormally large nose out of other people's bussiness.

Snape froze. Harry stared, dumbstruck, at the message. But the map didn't stop there. More writing was appearing beneath the first.

Mr.Prongs agrees with Mr.Moony and would like to add that Professor Snape is an ugly git.

. . .

Snape pointed at the parchment, on which the words of Messrs. Moony, Wormtail, Padfoot, and Prongs were still shining. An odd, closed expression appeared on Lupin's face.

Harry potter and the Prisoner of Azkaban

Here, Snape found out that Harry was possessing this magical map, and called Lupin to ask him what this is. It turns out later in the book that Lupin is, in fact, Mr.Moony, which means he was one of the four who made the map which teases Snape. But Harry knows none of these yet.

In this situation, what does "closed" expression mean? I thought from the context that Lupin seems to or wants to hide something but not sure what exactly it means.

Can anyone help me with this expression

Answer 1

A person's facial expression normally tells you quite a lot about what they are thinking: whether they are happy, confident, enthusiastic, and also how they feel about the person that they are looking at or talking to.

When somebody wears a closed expression, it means that they are deliberately hiding their emotions because otherwise they would give away information that might be valuable to other people, for example weakening their position in negotiations. You can also call it a poker face- so-called because you don't want your face to give away information about the strength of your hand.

On the other hand, if somebody wears a blank expression, it means that they don't know what to think, and this shows on their face.