https://www.cheatsheet.com/entertainment/cheers-has-to-recast-a-role-when-a-nervous-guest-star-bolted.html/

'Cheers' writer Ken Levine reveals the season 11 episode that caused a guest star to run away before filming his scenes. They recast a future 'Frasier' star in his place.

I think it's fine we don't know who the actor was - it sounds like he had something serious, and Ken is correct not to humiliate him.