Maybe the "switch" doesn't happen until the end
Ok, so I'm not going to bother spoiler-tagging an entire thread. If you haven't played Ep5 yet, STOP READING THIS AND GO PLAY IT.
A lot of people are talking about the final scene and its implications about Faith and Nerissa. I've seen two theories get thrown around. One is that Bigby never met Nerissa; it was Faith the whole time. The other is the reverse: that Bigby never met FAITH, because it was Nerissa the whole time. Both of these theories have some problems that have been pointed out in other threads, so I won't re-hash those.
Instead, here's a third theory I haven't seen suggested yet, and it's a much, much simpler one: Bigby met BOTH Faith and Nerissa. Everything happened exactly as it appeared to. Unseen by the audience, Faith has somehow been restored to life by the end of the season. For reasons of her own, she chooses to glamour herself as Nerissa and talk to Bigby one final time, giving us the final scene. While she's there, she tells him the rest of the story as if she really were Nerissa, even though the things she's describing happened to the real Nerissa, not to her.
This eliminates the problems of pre-season murders and the problems of convoluted glamour-swapping, and greatly simplifies the story. However, it's not without some flaws of it's own: First, how did Faith come back to life? Literally losing your head seems to me to be something that would be pretty fatal, even to a Fable. Second, why does Faith bother glamouring herself as Nerissa in the end? All of her enemies would appear to be dead.
I find these problems easily solvable, however. With the first one: who cares? Miracles and magic are one of the staples of the setting. Comic book characters come back to life all the time. I don't find it at all hard to believe that there was some magic at work we don't know about which allowed our beloved Donkeyskin Girl to rise again. It's a mystery that doesn't really need an explanation, at least not for me. As for the second one, there are a few possibilities. One is that she doesn't know her dad is dead, and wants to stay hidden from him by continuing the pretense of her death. Another is that she has other enemies we don't know about, and so being thought dead is still to her advantage. But I like this last one best: she just doesn't want the fanfare of suddenly revealing to the world that she's still alive. Her story is that of someone who disguises herself to get away from unwanted attention. She's simply doing it again in order to talk to Bigby one more time. She can't resist giving Bigby a little hint (her repetition at the end of her iconic line from Ep1), but she knows he's intelligent enough to deduce from her disguise that she doesn't want the world knowing she's alive, and so he'll keep her secret.
To me, this theory makes the most sense right now. But I'm sure I've overlooked some critical problem with it--which is, of course, why I posted this. So criticize away! 
Comments
No sorry but this is probably wrong. If they weren't the same person throughout then they wouldn't have had the same body language and said very similar things.
Spoiler in the title?
A problem one might bring up is how Faith knows what happened at the trial, but one possible idea could be that Nerissa knows Faith is alive, and she told Faith about the events. On the flipside, Faith could tell Nerissa about her conversation will Bigby so she isn't in the dark on it. It would also make it easier for Faith to get a glamour of Nerissa.
It's possible. Faith was obviously close with her, and she seemed like a smart girl who could pick up on those sorts of things. Nerissa no longer being bound by her ribbon changed her mannerisms a good bit as well.
Nerissa is hiding something. And Bigby realized it at the end, but it's still confusing as to what it is.
Don't forget that if Nerissa was killed while being glamoured as Faith, it would wear-off just like it did with Lilly.
Lily's doesn't just wear off due to death, it goes away same as Auntie Greenleafs did in her apartment... it goes away when the tube is opened.We never found Faith/Narissa's body
I hope that somehow Faith is Nerissa and not the other way around. Although I don't see how this could be too possible.. Who was glamoured to look like Faith when we found her head? This leads me to believe it's been Nerissa the whole time, which makes me sad. I liked Faith way more..
What you're saying is true, but the game made it a point to bring attention to the familiarities between the two. I think the 'Nerissa glamoured as Faith' theory is the most likely.
What he means is that eventually the head would revert back to it's true form. It would wear off.
Regarding what people are seeing as similar mannerisms between the two girls: I did not notice anything like this. The only parallel I noticed when playing the game was the line "Do you like my ribbon?", and I interpreted that to mean that Nerissa was simply trying to tell Bigby the same thing Faith was--that the ribbon was the source of her inability to directly answer his questions. I'm not sure what other mannerisms people are referring to. I suppose one could see their facial expressions and gestures as being similar, but one could say the same thing about Snow White, so...idk.
ab123--you think so? I was trying to be vague but maybe "girl switch" was a little too specific. If so, I apologize to anyone I may have spoiled this for. Perhaps I should have just written "switch". EDIT: After posting this I discovered I can still change it, and did so. Hope that is vague enough to not spoil anyone and specific enough that those who've finished the game will know what I mean.
PowerfulStache--ah, you bring up a possible problem that hadn't occurred to me. However, I think your solution to it is very viable. It seems reasonable that as soon as Faith came back from the dead, she met with Nerissa (one of her best friends, remember) and was thus caught up on everything. In fact I can even see a scenario in which Nerissa says, "I have to go tell Bigby blah blah blah" and Faith says, "Oooo, let me do it! I'll pretend to be you..."
To those pointing out the "glamour wear-off"...much as I'd love to use that as evidence to support my theory here, we can't. If I interpreted things correctly in the game, jdubs is correct. Lilly's glamour did not wear off until Bigby opened up the glamour tube, much like Aunty Greenleaf's wore off for the same reasons. So as long as that hidden tube stays closed, a glamoured head would continue to be glamoured. Maybe there is some kind of time limit on the glamour tubes, but if so, we don't know what it is, since Bigby forcefully ends the only glamour tube glamours we see in-game. So regardless of which theory is accurate, the "Faith" head would continue to look like Faith's head.
Regarding an eventual answer in a potential season 2...personally I think we will never get an answer, even if we do get a season 2. That scene really felt to me like it was the last "Nerissa/Faith" scene we were going to get. I don't think we'll even see either character in a second season. I think Telltale left it intentionally vague so that we could all come up with our own theories--a technique which, to judge from these forums, succeeded admirably
.
EDIT: A forum user named "Saltiye" pointed out in another thread that the "Faith" head we see has bruises and injuries in exactly the same places as we saw "Faith" get hurt by the Woodsman. This would seem to indicate that it was the same woman, which lends a lot of credence to my theory, since both of the other theories rely on the assumption that the head we saw did not belong to the woman we'd seen previously.