There are always real-world and story-related reasons for these sorts of decisions, and we can never know for sure which was really the deciding factor. But in this case, it seems more likely Diane died for the story.

When a character is written off, the real-world reasons are always harder to get at -- as with romantic breakups, everyone will say it was their idea.

Melinda McGraw, who played Diane Sterling in six episodes across five seasons of "NCIS," has had a number of larger roles since her character was killed off in 2015. That suggests she was at least all right with the idea of leaving -- she had her eyes on bigger things, including leading roles on Cinemax's "Outcast," ABC's "The Crossing," and, most recently, the CBS All Access crime series "Interrogation."

However, she appeared pretty infrequently on "NCIS," so it's not like her show obligations would have gotten in the way of those other plans anyway. That suggests the writers wanted her death to further the plot.

It certainly did that -- she was killed at the midpoint of Season 12, and it raised the stakes for both Gibbs (Mark Harmon, "Chicago Hope") and Fornell ("Joe Spano, "NYPD Blue") -- both of whom were Diane's exes -- in their hunt for the mad international mercenary, Sergei Mishnev (Alex Veadov, "Svetlana").

Unfortunately (for them, at least), this is often what recurring characters are for on serialized shows like "NCIS." These characters are around enough for viewers to form an attachment and to show how important they are to the main characters, and then are killed for shock value and to bring some emotional heft to the story.

If we just watched Gibbs and company do their regular jobs each week, chasing unconnected people for random crimes against the Navy, it would start feeling like we were just watching them cash paychecks. That is indeed how the show was at first, but once that got stale, the crimes got personal.

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