It's a great read, but the work-in-progress videos are even more interesting. Below is the first half of the "very first" animatic created by Valve's team. Valve went back to the drawing board, updated the models to the versions we've seen today, and came up with a new draft a few weeks later.

Developer Jason Mitchell of Valve notes that the model changes were partially due to facial animation issues.

"One of the reasons that the Survivor models were redesigned was that the original meshes did not hold up well under the demands of the facial animation that was required for both the game and the intro movie," said Mitchell.

The romantic tension between Zoey and Francis also didn't sit well with playtesters.

"Over the course of the next couple of weeks of refining the story, we found that the romantic tension between Zoey and Francis that you see in this version did not playtest well, as it proved to be a distraction from the gravity of the Survivors' desperate situation," added Mitchell.

The final version of the intro can be seen below, finished over a span of three and a half months: