As fashionable primetime trends go, death is becoming TV’s new black.In their efforts to push the envelope and keep viewers off-guard, dramatic series have inflicted an unusual assortment of casualties, highlighted this fall by ABC’s “Lost,” which killed another member of its inordinately large cast; and Fox’s “Prison Break,” where the escaped inmates’ life expectancy has plummeted since they cleared the prison walls. Still, that’s hardly the end of it. “Desperate Housewives” eliminated a supporting player at the outset of the November sweeps, “24″ got off to a rousing start last season by blasting one-time regulars — including Dennis Haysbert’s former U.S. president — and NBC’s new drama “Heroes” is teasing that the Grim Reaper will come calling. Admittedly, characters have died in television series before, and some came as memorable shocks — perhaps none more so than the much-debated demise of dragon-lady attorney Rosalind Shays, who plummeted down an “L.A. Law” elevator shaft in 1991. Historically, though, core casts from episodic dramas were relatively immune, even in perilous settings. The boys on “Bonanza” saw wives and potential wives come and go, but seldom caught a bullet anywhere but the shoulder. Indeed, the only sure-fire way to die was to hold out for more money, as happened to John Amos’ character — dispatched in an off-camera accident — after the third season of “Good Times.” Similarly, McLean Stevenson’s Col. Henry Blake on “MASH” went down in a helicopter crash (as in, “And don’t come back!”) after asking to be released from his contract. So why the sudden rise in TV mortality rates? Several factors appear to be at play. For starters, the stakes in serialized storytelling have risen, and so has the need for big moments within those narrative arcs. By telling more ambitious stories, producers also seem more willing to take chances and shake up casts — even if that inspires a portion of the audience to wail about it on the Internet, as happened among the “Lost” faithful who greeted the exit of Mr. Eko (Adewale Akinnuoye-Agbaje), the show’s fifth major victim, as if they’d lost a real-life pal. Another motivation might be the HBO effect, inasmuch as the pay net’s acclaimed dramas — beginning with “The Sopranos,” but also “The Wire” and memorably “Deadwood,” where Wild Bill Hickok checked out in episode No. 4 — operate in milieus where life is cheap and exhibit no compunction about eliminating prominent denizens. Finally, with networks significantly cutting back on movies and miniseries, series must deliver the kind of promotable flourishes relied upon to raise sweeps profiles. This November’s survey, in fact, is noteworthy for a glaring shortage of specials and stunts, reflecting both less emphasis on sweeps thanks to the advent of local Peoplemeter demographic data as well as a strategy that says it’s better to stick with regular programs than to disrupt established schedules. All this requires a certain moxie among programmers and producers, who realize they must gamble on letting characters die to keep key franchises plowing forward, as opposed to merely running in place, even if that means angering some viewers. As for actors these days, even in a hit show, it’s wise to look twice before stepping into that elevator.