In Episode 69 the Sages of High Wycombe very briefly discussed the diplomacy required when playing a character in a game set in a world that one has created. Professor Cule seemed a little startled to discover that Professor Bell_West has done that. I’ve done it at least eight times that I recall.
Two of the long-term players in my SF setting Flat Black, veterans of the first campaign and several others, each ran something in it twice. One tried two campaigns of Secret Service intrigue involving setting-busting developments; I played Lord Tacitus Arbeiter in the first (which was unsuccessful) and Lt-Major Miles Scrivener in the second (which worked okay). The other ran two one-off adventures of straightforward criminal investigation in which I played characters whose names I don’t remember. Those were both fine, though my character in the second was killed because of a representation problem in HERO System.
Three different friends have run campaigns or adventures in my fantasy setting Gehennum, one of who was one of the Flat Black GMs and also an original and long-serving veteran of the setting. I played Bluefin, Methlin, Albion, Ivrian of House Green and Scarlet, and Ivrian Immanides “Evander”. The campaign with Bluefin in it was okay, I think. The others were all unsuccessful for various reasons.
If I’ve distilled anything useful out of these events it is that if you are in a party where you know the setting better than the other players things become especially difficult if you play the person in charge. It’s better in that case to play a very capable expert in specialist role.
I haven’t done this as a player, but as a GM, I have had a party visit a setting that had been designed by one of the players, and in which I had played. I understood it reasonably well, and he found my rendering of it quite satisfactory.
This can become an issue in fantasy-historical settings – one of the players may have a lot more knowledge of the historical setting than the GM – so what does that player do and how does this impact the other players enjoyment?
The closest I came to this was DC Realtime, a campaign another GM and I collaborated on. The premise was that each DC superhero began their career in the year when they were first published, and aged realistically since then. (So when the campaign started, Batman was dead, Superman had some distinguished gray at the temples and was focusing on scientific research rather than crimefighting, and Wonder Woman hadn’t aged a day.) There were variations, of course: Superboy started his career in the late 1940s and turned out to be Superman’s illegitimate son, and the Legion of Super Heroes started their careers around 2960 and time traveled to visit 20th century heroes.
But anyway, each of us had a player character whom we ran when the other was GMing. I played Tik-Tok, earth’s first machine life elemental, and he played the original Captain Marvel. We both refrained from taking anything like a directive role, treating what we did as character parts rather than starring. It seemed to go fairly well; the campaign was well liked enough to last through two cycles, a total of about four years. (That was the one where I used White Wolf’s Midnight Circus supplement as a scenario; the sinister circus is a class motif in superhero comics, and it turned out to be really easy to adapt Storyteller to DC Heroes.)
This hasn’t happened much to me because I haven’t had any worlds published - the one I wrote up in enough detail for someone else to run isn’t available any more (and I don’t particularly want to resurrect it).
I did once try to put together a collaborative Cyberpunk 2020 campaign, but it turned out that to several of the people involved “collaboration” meant “I get to use your NPCs, you don’t get to use mine”.
I agree with @Agemegos that I’d want to play something quite specialised in such a game rather than a direction-setter.
@AndrewK, I set out to run a World War II campaign with two players who are very knowledgeable about it, and the answer was to bring them on-side: they’re competent role-players, they can tell me about stuff (before it comes up in the game) without using it as metagame foreshadowing, and we all have more fun as a result.