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She quickly followed up by adding, "Dude I'm not dead!!!"
Of course, there have been plenty of movies about people's lives that have come out before they were dead, so never say never!
In fact, Spears has hinted several times that she's working on her own memoirs, to presumably be published while she's still alive. Maybe that could be the basis for a biopic starring Millie Bobby Brown.
Considering Spears is still culturally relevant at 40 years old, Brown has plenty of years ahead of her to tell this story. It really does seem only a matter of time before someone will.