It'd have to be play-by-mail, which sounds glacial.
You have no idea. I took part in one such game as a GM. In eleven months we have accomplished less than during my typical two sessions.
Pros of pbm format:
- Charismatic and scholarly characters can actually pull these off. It's the little things, but a whole speech by wizard can get a lot smarter with one moment of thought or glance at the thesaurus.
- It's browseable and with 'paper trail', so even if you are confused you can always get back to what just occurred.
- If any of the players can draw, then even a pencil sketch can add a lot to the game.
- You can say that you aren't comfortable with speaking as the character in first person all the time, but there's no excuse from writing like that. It can help people with some social problems as well.
Cons of pbm format:
- Glacial pace.
- Combat gets hard to track if you don't add some map with updates.
- Some dialogues that would take ten minutes in a face-to-face game take a fucking week.
It's a good idea to make some sort of agreement along the lines of "one post/message per day with at least 200 words in it" and make it a point that a short discussion can be made in some instant messaging system and you will just paste it for all who heard it.
Or just play shorter sessions if the commute isn't a problem. I would argue that it's better to play for two hours via Roll20 or MapTool (if face-to-face isn't possible) than go to pbm format.