I thought of a trivia trope that might be great, not sure how to title it or if it'd be too "people sit on chairs", but I'd like to see a trope for animated shows where most of the voice actors do multiple characters, but then there's voice actors who only do one character. I don't think there's such a trivia trope.
There are animated shows where it seems each voice actor only does one character (it seems like most animated Disney shows do this, even when the voice actors generally do multiple voices on other shows, with Jim Cummings being the main exception, as he seems to do a lot of voices on the Disney shows he's a part of), but many tend to use the same actors for multiple roles, and then there's actors who just do one voice. Notable examples include Yeardly Smith on The Simpsons, rarely doing any non-Lisa voices (and when she does, the characters are meant to be very similar to Lisa), and William Conrad, the narrator on Bullwinkle. There were rare occasions when he did do other voices, but as noted in The Moose That Roared, he didn't do character voices othen because he didn't have many voices that sounded cartoony enough. A number of celebrity regulars on animated shows seem to also have only one character (such as Adam West on Family Guy).
And in a puppetry example, Caroll Spinney kind of counts, though it's not exactly a one-character performer, but for most of Sesame Street's run he's generally only been Big Bird and Oscar, while the other Muppet performers could do any number of characters. I think Noel MacNeil on Bear in the Big Blue House would count as well - I don't think he ever performed any additional roles, but he was likely too busy with the lead character to be able to do other roles.