Occasionally at the end of Arcade mode, I'll be able to fight Akuma/Gouken in the extra stage after the boss battle (with Seth).
Unfortunately, this doesn't quite so often. I've played through Arcade Mode, only for the character cutscene to trigger right after the Seth boss battle fight, and sometimes getting to Akuma. I've only fought Gouken once, and I have no idea how I was able to fight him.
How can I tell when I'll fight Akuma/Gouken at the end? Is there a requirement I need to do (take minimal damage, combos, etc.?) to actually fight them?