When writing a script, you want to make sure that you don't have two characters who seem to serve the same function. I'm sure you can figure out the most important reasons for this, the least of which not being that screen time is too valuable to have two characters crowding each other out by stepping on each others toes, dramatically speaking, that is. When this happens, usually one character is redundant and should be eliminated in the name of simplification.
So if you didn't see SWATH this weekend, allow me set the stage. Early on, we meet Snow White as a child, and in an early scene she's shown with a a childhood playmate named William, the son of the Duke. When Snow White's stepmother marries and then kills Snow's father, the King, it sets the stage for an invasion from the new Queen's army. The Duke and his son escape, but Snow is captured while William can only watch helplessly.
In true movie form, when we later rejoin these two as adults it's pretty well hammered at us that these two are still carrying pretty big torches for each other. Putting aside how unrealistic it is, it actually causes some problems for the film. It feels like that at one point this character was conceived to be the equivalent of Prince Charming. When word breaks of Snow White's escape, he puts himself at risk to find her, taking on the Evil Queen's brother. It's eventually Snow's feelings for him that the Queen uses to trick her into taking the poisoned apple.
The problem: Snow White already has a noble protector, the Huntsman. Initially brought in to recover the fugitive Snow White for the Queen, he soon becomes her reluctant protector. Later, after some circumstances I won't get into, he becomes even more devoted to looking after her. The Snow/William is asserted more than it's shown, but the Huntsman/Snow dynamic actually develops and deepens over the course of the film.
(I can't go any further without a big spoiler, so just know that you've been warned.)
In fact, after Snow White takes a bit of the poisoned apple and dies, it's not William's kiss that revives her - it's the Huntsman's. This pretty much provokes the question: what does the movie need William for anyway?
I read an interview with screenwriter Evan Doughtery (which I can't find now, so if anyone finds the quote I'm talking about, please send it), where he talks about how one of the big changes from his draft to the final shooting screenplay was that he envisioned being older, with a more paternal vibe to him. Once they cast the part younger, that led to producers to play up a bit more romantic chemistry between the two.
[Edit: This Slash Film interview probably does the best job of explaining it.
"In my original draft another slight difference was the idea that the huntsman was primarily a mentor figure with some hints of an unrequited love relationship with Snow White. As an example, like the original pitch I would give for this movie back in the old days when I first wrote it was. “It’s Snow White meets Luc Besson’s The Professional” where it’s like the huntsman is like the Jean Reno hitman character teaching Snow White to be this strong warrior and fighter.
"So he was much more of a mentor in the original and the process of the script evolving and then the casting, they started thinking about older casting for the huntsman character, like Viggo Mortenson, Johnny Depp, guys like that, which would have been more that mentor figure.
"But when Chris Hemsworth became available and it was right after Thor and he was so good in Thor. I thought Thor was really over the top and crazy, but really fun and I thought he was really good in it. There was no way you could cast Chris Hemsworth and not turn up the volume on the love story between the huntsman and Snow White.
"So now it’s much more of a 50/50 between huntsman as a mentor and huntsman as a potential love interest and that was a big change, which to be honest I was a little resistant too, but knowing a huge portion of movie goers go to see… I mean it’s just a classic element of any story, a compelling romance or a compelling love story or the hint of a compelling love story. I have since been won over by that, but I was a little resistant to that at first."
In doing so, William had a less unique function to fulfill. As the Huntsman swelled to fill space he wasn't designed to fill, he crowded out William. After all, what's more interesting, watching a relationship grow and change from nothing, or being filled in about an already existing relationship?
All things considered, it might have been cleaner to write William out altogether. Frankly, given the set-up with young William, I was rather surprised when the adult Huntsman didn't turn out to be an older William. When you get down to it, the only unique function William fulfills is that its his image that the Queen uses to dupe Snow White into biting the apple. It's a moment that works better when it's one beat in a film-long arc, but with little else to really give it resonance, a rewrite was probably in order.
So in your own work, look out for redundant characters, particularly as you rewrite. Characters often change throughout the creative process, so it takes a keen eye to make sure they don't impinge on someone else's turf.