Even before the movie, I thought about the feeling each character lends the show (or lends to the crew, the family) simply by injecting his/her personality into the mix. Specifically, I've thought about the innocence versus "experienced" factors. I've thought about this even more since Wash and Book's deaths.
The way I figure it, Kaylee is innocent. We can all pretty much agree that she would fall farther on the innocent side of the spectrum, and let it be known that I'm referring to the feeling of cheerfulness and "it'll be okay-ness" she radiates, not necessarily the fact that she's comfortable talking about sex (or doing it in the engine room). Know what I mean? This innocence is the innocence of the Big Bad Verse, the belief that the grand scheme of things is generally favorable to everyone, or that there's always a nice way to do things.
Which is where Wash fits in, for me. He's always doing one of two things: cracking silly jokes to lighten the mood or incredulously asking whether the crew can get something done WITHOUT using violence. He seems genuinely surprised that they would resort to violence at (almost) the first possible chance, and is always there to peep in with a "uh, do we have to shoot it?" comment. I love that about him. (I almost used the word "love" in past tense, but let's face it, he's a fictional character... he still exists... *shed a tear*)
Book, in my opinion, does not land on the innocent side. Clearly, the man has a dark past and is trying to atone for it. You can't be atoning for something major and still be innocent. Now, I suppose he gives off such warmth and such a feeling of safety (the refusal to harm anyone), that he satisfies the effects of innocence in some way on the ship, serving to make things feel "kinder", less dangerous, etc.. He obviously serves as the crew's conscience in many ways, as well, with his "special hells", and smoothing-overs of tense situations, and trying to calm people down during show-downs. But innocence itself? No.
Inara is also not innocent. She's just too wise and too.... TRAINED to be innocent. She also appears to be running from something, which rules her out as a character who's just taking a stroll through the 'Verse without a care in the world. She has other pacifying and civilizing qualities, much like Book, but the feeling she radiates is an experienced, even dark feeling (dark as in somber, not evil).
No one would argue that Jayne is not innocent. Or would someone?
River is the definition of a character whose innocence was torn from her.
Simon is far too aware of the ugly face of the Alliance.
Zoe is a war veteran--she's on the far side of the experienced spectrum.
And, of course, Mal. Enough said.
The point of all this is the following:
For the sake of this discussion, pretend there's a new Firefly show or a series of sequels.
What kind of character would you like to replace Wash (or Book) as a permanent crew member, based on the feeling they lend the crew, whether it be innocent, experienced, or some other aspect? I've heard that River is supposed to be the new pilot, but let's pretend she's not, and that Mal needs to find some other pilot (or shepherd!!!). Would you want someone who feels gentle, cheerful, fresh, undaunted, innocent, friendly...? Or someone with a heavy heart, a heavy conscience, a dark past, a brain full of knowledge or secrets or experience or tactics?
Talk amongst yourselves!
PS I want someone to replace the innocence lost by Wash. But maybe it would be better, dramatically speaking, to simply let the innocence be lost.