It would all depend on the desires and interests of any literary executors
or estates. The Ian Fleming Trust, for example, would bring down full
vengeance on anyone infringing on the 007 copyright. In this case, they know
there's money to be made--and further, they wish to control any use of the
characters Fleming created. A few years back, someone did a sequel to Gone
with the Wind and they had to do so with the blessings of the Mitchell
estate. In short, if you're dealing with known and recognizable
characters--and there are heirs or a trust of some kind-- they'd be
interested in what's going on if you're talking about publication of some
kind. Money might be one issue, but they often are just as interested in
protecting the legacy of a given writer.

If you're just talking about student essays, I don't know that anyone would
care. But, again, if you're pondering publication, you'd best check out who
owns what. It can be complicated--someone might own the copyright to
specific books and someone else the rights to license films, media
adaptations, etc.