In the film, Sally takes on three basic modes of personality: her "normal public" face -- the librarian, her unhinged coldhearted psycho mode, and, at home with her brother Ruby -- playful and childlike and gleefully twisted.
Everybody does this in their normal lives to some degree, though most of us not to the extreme Sally takes it. As to whether I have to juggle personalities like that, I have a busy busy busy life, so I wear many hats -- so many that, though I juggle through different mind sets, sometimes the transition is so quick, they blend together. And sometimes I'm wearing several hats at once, and boy does that get confusing. I'm a mother, I work full-time as a graphic designer for a flag and banner company doing layouts and designs on the computer, I freelance as an illustrator of book covers and children's books (almost always mixing media -- my favorite is pencil, oil paint, and whatever else works at the time), I occasionally find time to do a little "art for art's sake," I act (obviously), I dance (modern, jazz, bellydance and African), I take Jiu Jitsu (so that's my killer instinct getting some attention), I often do the costumes for shows / films I'm in, I help w/ scenic art, I help w/ props... etc. etc. etc.... If I could just cut sleeping out of my day, I might actually be able to get all that stuff done! I stay insanely busy, but I wouldn't have it any other way.