The Expanse

Tapxe said
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.

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.

Both shows are the epitome of what they do.
Farscape was the first show with genuinely alien aliens. Rygel and Pilot weren't just actors with forehead prosthetics. They were also well-realized individuals. You could read a line of dialogue and have a good idea which character said it. They all had their own voice.
The Expanse is on the flipside of that realism coin. The show is so devoted to realism that actual physicists applauded the effects and the way space itself is like another character.