Some of the Hardest People to Run Games For

Can you imagine trying to run a game for this prick?

In my experience, other DMs can be some of the hardest players to run games for. 

This isn't aimed at anyone in particular; most DMs keep their shit together when they're playing a character in someone else's game. If anything, it's a reminder to myself for when I play in someone else's games.

It makes sense: people who run their own games tend to be heavily invested in roleplaying games as a hobby, they're often the people who like building their own worlds and settings, and they tend to have strong feelings about how game systems and mechanisms should work.

The thing is: I'm not sure you should do anything differently when you have a fellow DM playing a character at your table. The way you run your game is a series of choices you should make about the kind of game you want to have; people will either like it and keep playing in your game, or they'll hit the bricks and find a game more to their tastes.

The things is, part 2: If you are a DM who is playing a character in a game that someone else is running, I think it's a fine idea to let go of your preconceived notions and go with the flow of the game as it is being presented to you. There should be onus is on players-who-are-also-DMs to not to be difficult at the table--because, frankly, you should know better.

My Advice for Players-Who-Are-Also-DMs

• If you are a DM, you likely have some strong opinions about what game systems or even editions are best, what kind of mechanisms are most productive for a certain style of play, or how play in general should proceed. That's fine, but when you're a player do not interject your disappointment that the game you're playing doesn't work the way it would if you were the one running it. You aren't running the game this time; it's not your purview to make that call as a player.
• You can, of course, ask for rules clarifications and try to meet the DM halfway, just as any player at the table can. But if you approach it as an exercise in "Well, that's not how I do it..." you're likely making the game less fun for everyone at the table. You may truly believe in your heart of hearts that games should have a wound system, extensive critical hit charts, damage-reducing armor, and rules for infection--but if the DM is just using plain old abstract hit points in their game, those feelings are best kept to yourself.
• Also keep in mind that the world you're playing in is not your own, and it might be overreaching to assert authorship over it without checking in to see if that's cool. Many DMs love to homebrew their own bespoke settings, which is a great, hallowed pastime. What isn't great, however, is when a player-who-is-also-a-DM tries to take over someone else's setting by asserting background details, making up content unasked for, and generally trying to steer the setting from the backseat. 
• As a player, you already have an important piece of world-building to attend to: your character. The world around your character is essentially the DM's character. You wouldn't want anyone else making up the details about your character, right? Extend the same courtesy to the DM's world. 
• Some DMs like for players to "co-author" the setting with them to varying extents. The best policy is to ask how much input and what kind of input the DM might want from you. If the answer is "none," accept that! It's far better to ask if a contribution you want to make fits the DM's setting, the adventure they've devised, or the tone of the game they're running for you than to simply tell them that this new thing they hadn't accounted for exists. 
• You know how you like your players to be excited about the game? Be that player when you play in a fellow DM's game! Don't take it over or commandeer the game, just be a player who wants to work with everyone else at the table to have a good time.
• When you're taking the role of a player in someone else's game, the passion you have for RPGs is best spent in being the exemplary player. Be the player who shows up on time, the player who plays to the hilt, the player who passes the spotlight graciously, the player you'd love to have at your own table.