Fiction can only be used as prior art when what you see (or read about) is all there is to it, such as rounded corners.

It makes sense for fiction to be used as prior art in something like the rounded corners case, as the prop in question basically was an implementation of that patent in real life. Even though it isn't housing any real electronics, the plastic casing itself still exists, and simply putting some electronics inside doesn't make it a sparkly new invention.

It works less well when there are details in the implementation that aren't covered in said fiction or hand waved away with The Force or something. The sliding doors in Star Trek would be an example, as although the doors are seen to slide, you can still patent a mechanism that makes this effect possible.