And yet, I'd forgotten it. I'd argue that it's actually pretty easy to switch a character like that when you have the person right there to show you how and have been working with them for 30 episodes.I pretty much disagree with everything you just said. The episode where they all switched bodies was excellently acted. If people could actually swap bodies they wouldn't have acted any differently, that's how good they are. That's just one episode.
Adam Project comes to mind. A child actor was able to perfectly imitate Ryan Reynolds as a child. Not exactly a high bar for a kid who grew up with Deadpool and ... well Ryan only has the one character, but a preteen novice actor pulled it off.
Farscape also has the problem of I didn't watch it back when the effects were more cutting edge. They felt really cheesy and it hurt the believability of the show imo.
Like I said ymmv.