If the actors had not been able to nail the characterization, the movie would have flopped. Movie-goers who were even mild Trekkies would have balked. Choruses of "Kirk would have never said that" and "Spock would never do that" would have resounded through theatres everywhere. Instead, we were treated to a resurrection of some beloved characters. We could smile and say, "OMGosh! That so sounds like Bones" and "Scottie always did that!" Even Majel Barrett's (may she RIP) voice as the Enterprise computer was a pleasant familiar.
Of course, the movie wasn't a success because of the characterization only. The score, cinematography, plot and special effects all combined were also key, but all these could have been flawless and, had the characterization flopped, so would have the movie.
Characterization is just as important in books. If we're meeting characters for the first time--as is usually the case--by the end of the story we should feel as though we know them. Halfway through the story, should they do or say something that's out-of-character, their portrayal to that point should have been so vivid that this pull from their norm should startle us--even if it's done on purpose and for effect.
And, characterization relies heavily on keeping a tight point of view. So, it's imperative for authors to know their characters inside and out--even those little things that will never appear on the page, And then, as you're writing, remember to mentally "become" your POV character so that if you do happen to write something that would make the reader say, "Hey, John Hero would never do that" you'll recognize it first and go about editing so that your characterization remains flawless.