Kate Winslet, who starred in The Reader, used her German accent beyond the cameras.
According to Winslet, who used a German accent for her role in The Reader, her kids told her, "Mum, just be plain. Don't do any funny stuff like voices. Just be regular."
Of course, this is more lighthearted than some entries, but even her kids thought she was being extra. If using the voice helped her perfect the accent to the best of her ability, good for her, but not during story time!
Me too, Pal.