Now some movies do use focus groups, the Dark Tower which is currently being widely panned by reviewers (17% on Rotten Tomatoes) is apparently one of them. But these reviews tend to take place so late in the process that the fixes that are made to improve the movie may make it worse, and may have in this case.
So, you can understand the fear particularly if when someone tries to address it, the fix may make the movie worse.
We Have The Technology To Fix This
It is what you do with it. Particularly with a movie which comes together in parts and often is created from a script which in turn came from a far longer book the opportunity to screw up is almost certain. Generally, what separates a good movie from a bad one is a combination of skills, focus, and the ability for the director to have a decent vision of what a successful outcome will look like.
What surprises me is how often directors seem to guess at this. You could certainly survey readers of a book and find out what parts of it they would particularly love to see on film providing you with a better sense of what to keep and leave out. Using a game engine, you can even do a pretty good job of creating an emulation of what the movie will end up looking like with the lines in place before firing up the first camera or drawing the first real cell. You could then test the result to see if the vision works with an audience. This might help you with actor selection as well because you could digitally change out the actors in the rendered trial movie and tune it. Here is an example of the amazing quality you can get out of a game engine, in this case Unity.
Wrapping Up:
