You know those two pairs of hash marks that surround the dialogue in your manuscript? Some people call them double quotation marks. I call them shields. Why? Because they deflect the rules of grammar as surely as shields deflect photon torpedoes away from the Starship Enterprise. When characters speak in a piece of fiction, the only real rules are clarity and believability. If your character would mix up lay and lie when she speaks in her fictional world, you are right to record her grammar gaffe in your manuscript. It’ll be most believable to readers if she’s frequently casual with her grammar, but that’s not required. Plenty of people in real life get lie and lay mixed up.