Without a doubt, the best acting by in the series is done here by Eliza Dushku. Long story short (I never get tired of saying that), Mayor Wilkins leaves Faith a device that will allow her to swap bodies with someone else, and of course, she swaps with Buffy. This is actually a 2 parter, with This Year's Girl preceding the episode, containing Faith's awakening, finding the weapon and battle with Buffy. That episode ends with Faith having just body swapped with Buffy, but no one except the audience is aware. Mind you, the only way you KNOW they swapped is because when Joyce asks "Buffy" if she is OK, after knocking out "Faith", Faith-in-Buffy responds with a sinister 5 by 5.
Eliza Dushku captures Sarah Michelle Gellar's mannerisms and speech patterns perfectly. The scene where Buffy-in-Faith goes into Gile's apartment, trying to convince him that she is not actually Faith is the best of all. The dialogue goes as follows:
Giles: "Look, I know what you are going to say, and..."
BiF: "I'm Buffy."
Giles: "All right, I didn't know what you were going to say, but that doesn't make you any less crazy. If you are Buffy, then you will let me tie you up without killing me until we find whether you are telling the truth."
BiF: "Giles, Faith has taken my body, and for all I know she's taken it to Mexico by now. I don't have time for bondage fun. Ask me a question, ask me anything."
Giles: "Who's President?"
BiF: "We're checking for Buffy, not a concussion. Giles, you turned into a demon, and I knew it was you. I mean, can't you just look in my eyes and be all intuitive?
Giles: How did I turn into a demon?
BiF: Oh, 'cause, uh, Ethan Rayne. And-and you have a girlfriend named Olivia... and you haven't had a job since we blew up the school... which is valid lifestyle-wise. I mean, it's not like you're a slacker type, but - Oh, oh! When I had psychic power I heard my mother think that you were like a stevedore during sex. Do you want me to continue?"
Giles: "Actually I beg you to stop."
BiF: "What's a stevedore?"
I can't figure out how to close this post out.... so I will leave you with Faith in Buffy talking to spike:
Spike: "You know why I really hate you, Summers?"
Faith-in-Buffy: "'Cause I'm a stuck-up tight-ass with no sense of fun?"
Spike: "Well, yeah, that covers a lot of it."
Faith-in-Buffy: "'Cause I can do anything I want, and instead I choose to pout and whine and feel the burden of Slayerness? I mean, I could be rich. I could be famous. I could have anything. Anyone. Even you, Spike. I could ride you at a gallop until your legs buckled and your eyes rolled up. I've got muscles you've never even dreamed of. I could squeeze you until you popped like warm champagne, and you would beg me to hurt you just a little bit more. And you know why I don't? (pauses) Because it's wrong."