Here's why Guy Williams joined the Cartwright family on Bonanza and quickly disappeared
The Westerns tried to switch a son for a nephew at its peak of popularity.
You hear the term "prestige television" thrown around a lot these days. In many ways, the small screen has surpassed the cinema in terms of critical acclaim. Half a century ago, this was certainly not the case. Television was seen as inferior to the high art of film. This was not just an opinion held by audiences and critics — many of the people working in the medium felt the same way, too.
This is why you often come across cases like Robert Reed of The Brady Bunch. Reed was a graduate of the drama program at Northwestern University. He studied at Royal Academy of Dramatic Art in London. He performed Shakespeare in New York City. So it's easy to understand why Reed deemed himself a "serious actor" and felt that some of his Mike Brady material was beneath him. After butting heads with creator Sherwood Schwartz, he even famously refused to appear in the series finale, "The Hair-Brained Scheme."
Pernell Roberts was another thespian who honed his skills on the stage. In fact, like Reed, Roberts performed with a troupe called the Shakespearewrights. Classical theater was his forte. Thus, he was perhaps the grumpiest one on the set of Bonanza.
As a stage actor, Roberts was used to hopping from production to production, doing a little Eugene O'Neill here, a little Macbeth there. The notion of playing Adam Cartwright for 200-plus episodes made him antsy.
Unhappy with his job, Roberts looked to exit the Ponderosa. His opportunity came in season five with a love interest. Enter Kathie Browne as Laura Drayton. Adam and Laura met in "The Waiting Game" and their romance blossomed over the course of four episodes throughout that season.
Meanwhile, the producers of Bonanza were plotting out Adam's replacement. Enter Guy Williams as Will Cartwright. It would have perhaps been too out-of-the-blue to suddenly introduce another Cartwright son to the clan. Well, to be honest, this was 1960s TV and stranger things happened. In fact, Bonanza did indeed introduce a wayward Cartwright son the prior season in "The First Born," when Little Joe's long-lost elder brother Clay (Barry Coe) blew into town.
So, anyway, cousin Will Cartwright was written as Ben's nephew. That seemed more believable for a permanent cast addition. The producers snagged a dashing hero to play the newcomer. Guy Williams was widely known and beloved as Walt Disney's Zorro. This was all happening as Bonanza sat atop the ratings as television's most-watched show.
The transition from Adam to Will was meant to happen in "Triangle." The writers crafted an exit for Adam, leaving the possibility that he could simply ride off with his new lover. But in the end, the episode that was constructed to show the departure of Adam turned out to be the goodbye for Will. After five mere episodes, Williams vanished. He was never seen again nor mentioned.
Likewise, "Triangle" would be the last time we saw Laura Drayton, as well. In many regards, "Triangle" is the episode upon which the entire future of Bonanza swung. It would have had an impact beyond the Ponderosa, too, because if Williams had stayed on Bonanza, he likely never would have played John Robinson on Lost in Space.
So what happened? Well, the short version is that Pernell Roberts changed his mind. He stayed on the show for its sixth season. Not seeing a need for both Will and Adam, the writers erased Will. There are also reports that fans just did not like Will Cartwright.
But Roberts ended up leaving the show anyway after the sixth season. By that point, Bonanza felt it wise to leave enough alone and not introduce another replacement Cartwright.
Well, until honorary family members Candy Canaday and Jamie Hunter showed up in later seasons. This was classic TV, after all.