If this episode isn't the best in the series, it's certainly in the top-3 as one reviewer here stated, no question. I'm always into psychological thrillers, or mindscrews as they say, and this one was so well done I don't know where to begin.
Certainly the writing was more than solid, the acting top-notch. Johnson, who will never be confused with an Oscar winner, did one his best roles ever as he took us down the path of obsession and psychosis. This might be the beginning of Crockett's arc which culminates in his amnesia toward the end of season 4 where he believes he is, or rather becomes, his alter-ego, the ruthless Sonny Burnett. If this episode isn't he beginning of his arc of mental problems, it's certainly a catapult to it, because Crockett clearly had lingering issues at the end.
A great movie or show always requires a top-notch supporting cast. Jack Thibeau's Lt. Gilmore was incredible, and Vincent Caristi's Shadow was outstanding. For as good as Johnson was here, this episode would never have been as good without Thibeau and Caristi. Only possible critique is that John Diehl's Zito wasn't in it. But Diehl was on his way out of the series so I suppose his absence is understandable.
We can also mention Jan Hammer's scores, they were perfect for this episode. Then again, Hammer rarely got it wrong, if ever. And the fact they aired this on Halloween, you can't get a more perfect timing.
Season 3 had a smattering few hints that the show was downgrading from the consistency of the first two seasons, but Shadow In The Dark isn't one of them. Not by a long shot.