...Behind the scenes, John Drew Barrymore was originally cast as Lazarus, but on his second day of filming he went to lunch and never came back. He just abandoned the shoot.
I seem to recall reading that he just never showed up. I tried to look it up in Inside Star Trek but the index is incorrect about the page numbers concerning Barrymore.
EDIT: Correction. I found the quote.
HERB: ...the morning he was scheduled to begin work, November 16, 1966, John Drew Barrymore disappeared, ignoring his role, his moral obligation, and his contract. Calls to his home, his agents, his publicist brought us no insight or explanation. They didn't have a clue. The son of the famous John Barrymore had skipped out on Star Trek.
Inside Star Trek
No mention in that section about Brown having difficulties with the regular cast, just that the show ran a day over schedule.