The sad thing about fungibility is that while most people wouldn't consider other humans to be fungible commodities, most of the time, on accounting books, they are treated this way. A person who can do a task that any other person can do is considered fungible by the company, because if they were otherwise indisposed, anyone else could fill in without altering the workflow. As a counterpoint to this, most actors would not be considered fungible, because if they die halfway through filming the movie, you can't just change lead roles... BUT, they can be considered fungible when they are not currently filming and are merely prospective employees; for example, why pick Tom Cruise over Christian Bale, or Ben Afleck? It doesn't matter if one of them becomes unavailable because they can all play similar parts.
To illustrate my point, here's a handy chart:
Yes, all that nonsense was just to make a bad joke about a movie too old to talk about.