So, there are a bunch of games that implicitly or explicitly encourage the GM solicit players to help fill out world details. Asking them things like "what do you think it looks like" or "why do you think they want to help you". Basically, player improvisation on things other than the player character. Sometimes tangentially related to the PC, other times not at all.

(Note: this explicitly is different from session 0 player assistance with setting/world design , or even out of session solicitation for player ideas. I'm specifically talking about in-session improvisation.)

Have you done this, what was your players reaction, how did you get them to not only buy in to the concept, but to work it?

I've only tried it twice, a long time ago in the D&D 2e era, when such concepts were becoming popular. Both times players nominally bought in prior to the campaign. The first time the players were deer-in-the-headlights confused each time I gave them an opportunity. They tried, but never got beyond timidly offering vague idead. The second time three out of four players quit, because I was constantly "asking them to do the DMs job".

This experience has made me leery of play testing games that rely on such concepts. But I'm thinking of running some 13th Age, and trying to do it true to Heinsoo.