Let's say my friend is watching a movie on tv in his room, I walk into the room, watch a few scenes of the movie and ask
Is it supposed to be a comedy movie?
How is the expression different from the expression
Is it a comedy movie?
Does the first expression mean the same thing as second? Or does it have a negative tone like "is it supposed to be comedy a movie, because it does not feel like one"?
Yes.
In this particular sentence, asking if a movie is "supposed to be" a comedy implies that it doesn't appear to be a comedy. Presumably, it isn't funny.
It's common to use "supposed to be" more literally to refer to a desired condition, as opposed to an actual condition. Like, "What is the oil pressure in this machine supposed to be?" I may be observing a reading of the actual oil pressure, and I want to know if that is the correct oil pressure. Depending on tone, context, etc, I may be implying that I think that it is not the desired value, or it may be a straightforward question asking for information.
