The Verge and Joystiq both offer reports from a talk Obsidian CCO Chris Avellone held at GDC Online on the New Vegas DLC. From the Verge:

The creation of narrative probably seems like a process that requires the freedom and openness for the author to do whatever they want with the story they're crafting - but when the writing team at Obsidian Entertainment was charged with penning the four downloadable expansions to Fallout: New Vegas, they were given a hard limit for the amount they were allowed to write.

For all four DLC packs, there could be no more than 10,000 lines of dialogue total.

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"All of these things are important because they're going to save you time when you're recording at the studio," Avellone said, "because studio time is incredibly expensive, and the last thing you want is some actor spending five minutes debating this line with you trying to get it fixed, when you have 300 more lines left to read and you have no idea how you're going to get it done."

Bethesda requires a process called "text lock" for each of their titles, during which the script is essentially frozen for two weeks and checked for problems. Every line of dialogue is combed for errors, quest text is examined for logic flaws, voice sets are lined up against dialogue to make sure that voice overs and subtitles match. Everything is examined, from major NPC conversations to "barks," the reactive dialogue that characters shout during gameplay. Each character has 35 to 50 barks, Avellone said, which further ate into the team's 10,000 line total.