In the course of editing several
Green Hornet books for
Moonstone Books (as well as churning out a
series of action-adventure tales for Moonstone, Black Coat Press, etc.), I've found that one of the hardest concepts to identify, adhere to, and explain to others is Point of View.

I should start by noting that many different types of POV are acceptable, depending on what the writer is trying to achieve, and the scope of the story being told. However, for short, driving, action-oriented stories with a modern pulp sensibility, I feel strongly that a third-person subjective/third-person limited POV works best, and from an editorial perspective this is the feedback and direction I provide. I've also noticed that other writer/editors whom I greatly respect, such as
Christopher Paul Carey,
Matthew Baugh, and
Howard Hopkins, seem to be in alignment with this perspective (although I am not presuming to speak for them here).

(As an aside, there are some writers incorrectly think that proofreading is the same thing as editorial feedback, and since their spouse/mother/brother/best friend is an excellent proofreader, they really don't need to hear anything more from me or any other editor regarding their golden prose. But I digress; that's a diatribe for another time and place.)
Matthew has provided a great link to
Wordplayer.com which discusses POV from a screenwriting perspective, but is also instructive for prose.
I can't believe I'm going to quote Wikipedia, but the Narrative Mode section which
discusses third-person subjective/third-person limited POV is helpful.
Third-person, subjective
The
third-person subjective is when the narrator conveys the
thoughts, feelings, opinions, etc. of one or more characters. If it is
just one character, it can be termed
third-person limited, in which the reader is "limited" to the thoughts of some particular character (often the
protagonist)
as in the first-person mode (though still giving personal descriptions
using "he", "she", "it", and "they", but not "I"). This is almost always
the main character—e.g., Gabriel in
Joyce's
The Dead,
Nathaniel Hawthorne's
Young Goodman Brown, or the elderly fisherman in
Hemingway's
The Old Man and the Sea. Certain
third-person omniscient
modes are also classifiable as "third person, subjective" modes that
switch between the thoughts, feelings, etc. of all the characters.
This style, in both its limited and omniscient variants, became the
most popular narrative perspective during the twentieth century. In
contrast to the broad, sweeping perspectives seen in many
nineteenth-century novels, third-person subjective is sometimes called
the "over the shoulder" perspective; the narrator only describes events
perceived and information known by a character. At its narrowest and
most subjective scope, the story reads as though the viewpoint character
were narrating it; dramatically this is very similar to the first
person, in that it allows in-depth revelation of the protagonist's
personality, but it uses third-person grammar. Some writers will shift
perspective from one viewpoint character to another.
The
focal character,
protagonist,
antagonist,
or some other character's thoughts are revealed through the narrator.
The reader learns the events of the narrative through the perceptions of
the chosen character.
In our storytelling, through the use of
third-person subjective,
the reader is "limited" to the thoughts of some particular character.
Perspective can shift from one viewpoint character to another, but when
that happens, a scene break should be inserted. POV can alter
scene-by-scene (one scene from The Green Hornet's POV, the next from Mike
Axford's, the next from the criminal's POV, then back to The Green Hornet, or Kato, etc.), but mixing POV within a scene should be avoided. This is a matter of style, but one I consider to be very
important and to which I hold writers during the editing/revision process.

My novel
The Evil in Pemberley House (written with
Philip Jose Farmer)
is truly a third-person limited novel. There are no scenes, absolutely
none, that are not told from Patricia Wildman's POV. In strict
third-person limited, a novel or story could actually be (re)written in
first person and it would work.
Expanding on that thought,
regarding third-person subjective (i.e. multiple character POVs, each of
them in third-person limited, with scene breaks, as I've described
above), each scene could hypothetically be rewritten as a first person
scene for that character. The result would be an odd story and I don't
suggest anyone actually do this, but as a self-editing experiment (and everyone does extensive self-editing before sending a first draft to your editor...
right?), you can
see that if a scene is rewritten in first person, that first person narrator would
never narrate events about which he or she is not, or
cannot be,
aware. The same thing therefore applies to writing that same character
in a third-person subjective (and/or third-person limited) POV.

A simple way to conduct self-editing for POV violations before blissfully attaching your first draft to your editor and clicking "send" is to read a scene aloud to yourself, substituting first person "I" for for your third-person subjective "s/he" POV character. If the character narrates something they do not, or cannot, know about, then you've got a POV violation.
Once you get a handle on this (and I certainly can't claim to have
"mastered" it; writers, experienced and inexperienced, violate POV on a regular basis), POV violations will jump out at you all over the place
in your reading, and will drive you nuts. I am loathe to go back and
read any of my early
Tales of the Shadowmen stories, before I began to grasp POV, for fear of tossing them aside in disgust.
Or being compelled, in a very OCD way, to immediately rewrite and revise them!