Authors often create characters who are perfectly suited for the story that is being told. The brilliant detective is put to the task of finding out whodunnit, the brilliant surgeon must operate on the life-threatening tumor, the brilliant commando must rescue the hostage.
If these characters have suitable antagonists to oppose them, then enough conflict can be generated to make following along on their adventures engaging enough that readers will do it.
Fish Out of Water
But readers like to see characters both in and out of their element.
One of the best ways to define character is to plunge the character into unfamiliar territory.
So, what happens if the author promotes the robbery investigator to homicide detective? What happens if the life-threatening tumor must be operated on in a field hospital miles from the nearest blood bank? What happens if the commando is injured and tactical support for the operation completely withdrawn?
Now the character is tested in ways that will engage readers.