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It was probably not a surprising move for “Castle.” It just seems early for the ABC series to be bringing its two principal characters to a higher romantic level.

Arts and Entertainment Editor Bob Sokolsky says producers usually wait for their series to slump. Then they talk of the “sexual tension ” they have established for the stars, noting that viewers “have waited all season” for the love scenes.

But Sokolsky sees one flaw in the formula: It seldom works.

And he points to the problems series like “Moonlighting” and “Rhoda” experienced when they took that route.

But now, he says, viewers’ attention will be on “Castle” and he wonders whether this will also result in “an old story that has become too familiar.”