In which case, how does the switching work? Can you switch whenever you want, at a press of a button? Is there some limitation (a cooldown time between switches, a limited number of switches, only at certain places in the level, etc)? Do the characters share the same health (or is there even health at all)? Will one character vanish when you switch away from him, or will he just 'go idle' (e.g. the Lost Vikings)? Will levels be made so that you have to use both characters (again, think the Lost Vikings), or will it simply be encouraged?

And as for me, I'm an okay pixel artist (and getting experienced at flash spriting too), but I can't animate fire at all.