I don't think there is a clear demarcation, it is one of those things where it may be subject to interpretation.
I recall a Watchtower magazine article explaining how a couple was disfellowshipped for apostasy. They had a different interpretation of a matter and would not stop telling others about it. Later, the WT came around to the same interpretation, but the two were not reinstated. Their crime was not being right or wrong, but going against the directives of the GB. Presumably, they were warned to stop their behavior and ignored the warnings. So, which part made them apostates- disseminating a different view, or ignoring demands to stop?
Ray Franz, in Crisis of Conscience, explains that the GB took the approach that even thinking such thoughts was sufficient to constitute apostasy. Doubtless this was approach was taken specifically to deal with Franz and the others who were disfellowshipped for apostasy at that time. I don't think this is how they view it today, though they would probably warn the rank and file that entertaining such thoughts may lead to a bad outcome. In which case, thinking these things may lead to apostasy but not actually be apostasy.
And yet, apostasy would imply opposition to the WTS and the GB. The GB will refer to apostates as "opposers" sometimes. If you are a loyal JW, but you differ on a point or two, are you opposing the WTS/GB? Does it require action (telling others about it) or is it enough that you believe that they have it wrong? What if they change the teaching later? Were you an apostate for having it right when they had it wrong?