My girlfriend F/22 told me M/25 that she has thought about wanting me to talk to her in a mean tone someday in the future when she asks me to. We usually do this when getting intimate. Otherwise we are very affectionate and nice to each other. But this time she’s asking me to use a meaner or, for a lack of a better term, act in a shitty manner for a full day. I understand the fantasy element of her request and I’m willing to fulfill it although a little bit afraid of anything that could upset her. Since she sometimes gets upset about a joke I made and I always understood her reasons, apologized for it and explained what I meant.
How can I navigate this without under-delivering or going overboard on the execution?
EDIT: we have been together for almost 4 years. If that matters.