Shared with you
When Pernell Roberts decided to quit the series, newcomer---and former Zorro (1957) star---Guy Williams was brought in to replace him as the very attractive new character, cousin Will Cartwright, who was to become a permanent cast member. However, he only lasted 5 episodes due to Michael Landon's insecurities. Landon notoriously refused to work with any actor whom he deemed better looking and more charismatic than himself, so he had Williams fired. (Indeed, before filming Highway to Heaven (1984), Landon told the producers he refused to do the series unless his much older-looking friend, Victor French was brought in to play Mark. The producers had wanted a much younger and more handsome actor for that part.) Later, Roberts decided not to quit the series.
According to David Dortort, Michael Landon grew difficult during the last five seasons of the show: "Nearly every line, every scene, every set up, everything would halt for endless story conferences on the set, it got increasingly bitter toward the end."
During the filming of one episode at Paramount Studios, Lorne Greene was required to jump off a ship, into a water tank (known as the "B Tank"). Michael Landon recalled that when Greene did the stunt, he jumped into the water feet first and went completely under, but his hair piece came off and floated on the surface. Landon and co-star Dan Blocker watched to see what would happen next. After a minute, Greene's hand shot up out of the water, grabbed the hairpiece, and pulled it down. Greene emerged from the water tank, wearing his hairpiece slightly askew. He walked nonchalantly past the snickering crew, and went into his trailer without saying a word.
Pernell Roberts was not popular with his co-stars or the crew. He continually complained about the show, feeling it was poor quality and beneath him as an actor. According to Michael Landon, Roberts very rarely, if ever, spoke to him. As well as Creator/Producer David Dortort affirms in interviews with Television Archives.