1. When it's your turn, click watch mode.

You'll then get this message
2. Press space or right-click to pause.
3. Click list of comrade.
You think this is just for character's information and whereabouts? hahahaha

4. Click the name of the character you want to play, it doesn't matter who it is (except maybe characters from crime faction, you can't select 'em unless you did some editing.
5. If you choose a faction whose story is available/translated, you will get this message.

6. Congratulations, now you're playing as another character!
Don't worry you can still change character, I don't think there's any limit how many times you can change character.