I’ve been introducing my wife to the series lately, so in the past few weeks, we’ve watched the first 11 episodes (through “Trash”). And I have a hard time seeing that darkening trend. (And I can’t see the last three eps as evidence of such a trend either.)
Sure, we’ve got Niska (as discussed) in #10, but he was in the second episode, too, just not in quite as big a way - and of course the Reavers dominated #3. (IMHO, that episode - “Bushwhacked” - was the darkest ep of the entire series, and I’d characterize the pilot and the next two episodes as the darkest sequence of episodes.)
But back to the series v. the movie:
Sure, in the series as well as the movie, “Firefly is a dark fuckin’ universe.” But with rare exceptions, in the series, the darkness is out there somewhere, and the mood on ship, in between dealing with the threats of that 'verse, is one of wholeness and cheer. Think of the basketball (sorta) game at the beginning of “Bushwhacked.” Think of the conversation at the end of “War Stories” between Mal, Zoe, and Wash, which starts off with Mal telling Zoe that she’s got to sleep with him for the good of her and Wash’s marriage, and ends with Wash and Zoe headed out with the words, “We’ll be in our bunk.” (And this right after Mal’s being rescued from Niska. This says bad stuff happened, but back here, we’re still whole and well inside, and cheerful enough to kid about what we’ve just been through.) Think of the numerous relaxed conversations around the dining table.
In the series, Serenity is, as Patrick O’Brian would have Aubrey say, a happy ship. There’s no way around that. We’d all love to be part of those pickup games in the cargo bay, the conversations around the dining table, because they’re relaxed and happy and warm and full of trust and life and cameraderie.
None of that’s in the movie. Sure, there’s snappy lines, and the characters still trust each other underneath the tension, but the warmth and happiness and good cheer that were only briefly absent from the series are only briefly present in the movie. That’s a pretty major swing.
And it’s reinforced by the lighting. The difference in the lighting in the scenes inside Serenity in the movie and in the series is, almost literally, like night and day. I don’t usually think about the lighting and stuff in movies, but here we’ve got a situation where the same environment has been lit in two completely different ways, so it’s impossible to miss. And that’s a major part of the mood and ambience in each.