Methinks the only way you'll be able to play/roleplay a fallen cleric is by self-imposed rules and penalties.
First, based on your roleplaying style, decide what initiates becoming a "fallen" cleric. Maybe your cleric becomes fallen when your party's rep score falls below a certain point?
Then, decide what constitutes playing a "fallen" character. Maybe you lose your divine spellcasting ability? Or dual to a fighter and never cast cleric spells again, even though you'd technically gain them back after your PC's fighter level overtakes your PC's cleric level.
And if you ever "redeem" you cleric status (in whichever way you decide), then you may resume memorizing and casting your cleric spells. But, you never gain any additional spells, as an almighty penance, and constant reminder of the path that you should never stray from again (not to mention the fact that you're a dual class now too).
The beauty here is that there is no wrong answer here, because God(s) work in mysterious ways...
Why is it that whenever I finally get around to playing a new game for the first time,
I feel like playing Baldur's Gate for the second time...