A movie is a discrete unit of drama, fully planned and written before production starts. TV dramas and sitcoms were once a series of self-contained episodes that followed a familiar formula and ran all year long, and when they ended, they went into syndication (if the show was good). But TV shows today are more of a hybrid of the two, with fewer episodes overall, a continuing story arc, and sometimes rabid fans looking for a satisfying ending.
The answer to the question “Did you have a plan, or were you making it up as you went along?” is always “Both.” Gilligan’s Breaking Bad writers, for example, realized midway through writing season three that they weren’t happy with the story’s direction, so they killed off their two main antagonists and focused on a new one. Gilligan’s crew did a lot of this sort of thing in seasons three through five, and because they’re Carol Burnett–level masters of Acting Like They Meant to Do It, and equally good at cleaning up loose ends after the fact, the show hangs together better than it probably should.
Vulture looks at the way several shows wrapped up in the series finale, and speculates on how Breaking Bad's ending will affect its viewers.
It’s probably especially hard to write an ending for an anti-hero, like the ones on dark post-Sopranos dramas such as Breaking Bad, Dexter (which ends September 22*), and Mad Men (which enters its final season next year, and will compete with Breaking Bad at the Emmys), because a big part of such shows’ excitement comes from the dual pleasure of simultaneously loathing and cheering the protagonist.