This is the perennial question ... what should comes first? Plot? Character? Both? Neither?
I was thinking about "Monsters" on the walk back from work, did I think of the characters first? Or the plot? Or the setting?
It was after Buffy had finished that I decided I wanted to write something to replace it. We'd had Alias which I sort of lost contact with somewhere into the second series. Dark Angel which eventually got quite good before it was canned. There was the faint promise of Ripper, something British by Joss Whedon, Dr Who hadn't returned and neither Supernatural or Heroes had been thought of yet (well, they might have been thought of, but anything public was far in the future).
So I thought, something similar but different. Not supernatural, not superheroic in the "this defies basic physics" sense (I love superhero stuff, I'm no slave to physics, it was just my thinking process). Definitely with a kick-ass lead teenage female.
So I decide on mutation and wrote a TV series bible for it, with lots of world creation. The lead character came first, the setting came second and that dictated the plot.
For "One Night in Paris" it was different. The setting was dictated by a song. Some of the supporting cast were suggested by the same and other songs by the same band. I had a rough plot and then I needed lead characters to fill it. I had to answer the question: "Who would spend one night in Paris and meet a prostitute?" The character was dictated by the setting and rough plot.
For "Running" again it was setting, and the dictates of the production company, that lead to character and plot simultaneously.
So which comes first?
It doesn't matter. It's totally 100% irrelevant. I might even suggest that if someone thinks it matters then they haven't yet attained a professional writing attitude -- or they're trying to sell something.
What matters is characters the audience can care about in a situation that demands their attention. Something that generates an emotional response, because that is what audiences really want. They want to laugh, cry, get angry, feel loved and loving, and all that emotional stuff. That's what really counts.
And the road you take to get to a script that delivers those things is irrelevant, as long as you do get there.
What's on the turntable? "From Now On" by Supertramp, on their Greatest Hits compilation. Supertramp tell stories with their lyrics, well worth listening to.