Hollywood is an industry, and the old showbiz adage says that "the show must go on." Yet some actors are irreplaceable.

The unfortunate death of principle stars has forever been a fact of life in the world of television. What did evolve over the decades, however, was the manner in which series dealt with the passings. Sometimes, TV shows simply ignored the missing character. Or perhaps that person was explained to have moved away. In some cases, the show just could not go on.

The all too sudden death of Dan Blocker changed everything. The beloved man behind Hoss on Bonzanza held too big a place in the hearts of the cast, crew and fans to shrug off his absense or throw in the towel. After the 13th season of the iconic western, in the spring of 1972, Blocker died at the age of 43. The producers took the bold step of addressing the tragedy and working the death into the storyline of the upcoming (and soon to be final) season.

Though this was pioneering stuff for a television series, it was not the first time, nor the last, that a show would have to face such a dilemma. Here are a handful of other lamentable demises that happened during the production of favorite television series.

1

Nana Bryant on 'Make Room for Daddy'

The veteran had appeared in dozens upon dozens of films before taking on the role of Danny Thomas' kind mother-in-law. Early in the series, Bryant died and simply vanished from the show, with no explanation given. On the other hand, when Jean Hagen, who played Margaret, Thomas' wife, left the series shortly thereafter, it was written that she died off-screen.

2

John Hamilton and George Reeves on 'Adventures of Superman'

Hamilton had the significant role of Perry White, boss at The Daily Planet. When he passed in late 1957, producers planned to have the character away on assignment and replaced by his brother. This, of course, never came to be as Superman himself, George Reeves, died months later at the age of 45 under mysterious circumstances. The superhero would have to fight crime in reruns from then on.

3

Ward Bond on 'Wagon Train'

Like Nana Bryant, Bond simply disappeared from his show, well into its fourth season. Trail master Major Seth Adams was replaced with no explaination by Christopher Hale (played by John McIntire).

4

Henry Kulky on 'Voyage to the Bottom of the Sea'

Much about Voyage changed between seasons one and two. The show went from black & white to color, and the plot moved from Cold War spy themes with a touch of sci-fi to more outright fantasy with more frequent monsters. Additionally, Kulky died after the show's first year, and his character of Chief Curly Jones would have to filled. Enter Francis Ethelbert Sharkey. Read 8 more fascinating facts about Irwin Allen's sci-fi series.

5

Smiley Burnette and Bea Benaderet on 'Petticoat Junction'

The lighthearted rural comedy had the misfortune of two older residents of Hooterville dying, not far from one another. First was longtime country and western star Burnette, who was Charley Pratt on the sitcom, engineer of the Canonball train. More significant was the loss of Benaderet a season later, a.k.a. matriarch Kate Bradley, proprietor of the Shady Rest Hotel. Though at first the stories said that Bradley was merely "out of town," Petticoat alluded to notion of the mother being dead, with later mention of a guest staying in the "Kate Bradley Memorial Suite." Benaderet's void was filled by June Lockhart. Eerily, Burnette's replacement, Byron Foulger, died the day the final episode aired.

6

Dan Blocker on 'Bonanza'

In "Forever," the two-part season 14 opener written and directed by co-star Michael Landon, would be the first episodes withouth Hoss. The plots in the final year alluded to the death of the character. It is not coincidence that Bonanza failed to make it much further without Blocker.

7

Nicholas Colasanto on 'Cheers'

The saddest part of Colasanto's demise was its slow, oncoming approach. The actor's heart condition worsened throughout the third season, and his fatherly character was slowly scaled back until filming his final episode, eerily titled "Cheerio, Cheers." Still, it would not be until the first episode of season four that Coach's death was acknowledged, when the character's young pen-pal from Indiana, Woody, arrives in Boston only to learn the unfortunate news. Of course, Woody would stick around.