The term "over my dead body" is commonly used to show that someone is vehemently opposed to something. It's commonly used in conversation, either in a joking manner or as an ultimatum. The difference between usage is normally shown by body language and/or the method of vocalization.
Sometimes in literature it can be used as something ironic or for comedic effect.
Sheriff: You're coming with me to the jailhouse.
Desperado Jim: Over my dead body!
Sheriff fast-draws his six-shooter and fills Desperado Jim full of holes.
Sheriff: As you wish, Princess.
If used as an ultimatum, make sure you know the other person well enough such that they don't take you literally.
Iron Noder 2017