In your bog standard fighting game, even more so one in which the AI might take over from idle players, it should just be a little flag or similar somewhere in the game that you can tickle with a cheat to have the AI do something. I don't imagine it being too different for the average mario party title.
That said I am curious. I don't know how the game in question works but from the phrasing it sounds like the others can be kicked to AI. That says that they actually wrote extra code to prevent something. Now it could be that play testing revealed some people could be morons and press buttons and frustrate themselves but it could also be something like not enough CPU to handle everything. That said it is theoretically code already within a game that it already uses so no properly red flags at this point.
Searching for the code style flag is tedious but should not be too drastic. You would presumably start games and find out where the character info is (savestate, start a game with a character, search, revert to save, everything else the same but start with different character, search to see what is different, repeat until found, if you are really lucky it will all be near any collectables or score tokens which is an even easier search. You might get somewhere simply hovering or selecting a character in the menu but not starting a game, or it might be a complete wild goose chase). AI or not might well be somewhere near that. If not then time to search for the AI or not flag, same procedure but select one for AI. Once found for player 2, 3, 4 or whatever then 1 is likely nearby with a similar memory setup.
The proper hacker way is to see what you can do on the menus, and then what is stopping you from selecting something. At that point either set it yourself or remove the block. While potentially quicker, and definitely a whole lot more certain to get/know you are always working towards a result (even if you ultimately don't like it), then this is very much a versed hacker approach where you can learn to find simple memory cheats in 10 minutes (less if you already know the basics of programming or digital logic).
Ultimately if it is a purely aesthetic thing then meh. If you want to make a glorified screensaver then OK.