Many times the young'uns weren't in the mood for it and demanded just my "normal" voice. Other times, if they wanted a story I hated, I would open the book, and start "reading" a completely different one that I knew off by heart and liked better. The other stunt I'd pull with memorized stories was keep reciting and to "forget" to turn the page and see how far I'd go before they noticed and start flipping to catch up. And when they got bigger, I'd give them the choice - "Who's reading? You or me?"
I'd say it's the thing I miss the most from the preschool years.