I have only watched maybe 4 or 5 directors commentary on movies in my life. I loved 28 days later, so one day I was like "Fuck it, lets hear the directors talk about it." and it was shocking how bad this movie almost was. Basically, at one point in the movie they just pause it and start talking about how the whole last 20-30 minutes of the movie had to b changed. Then it goes into a story board of what was supposed to happen, which was the main character does a total blood transfusion on the girls dad, so that he becomes infected, and its a happy ending for the girl and her dad. They had filmed a bunch of it, and then one day, some random crew member went "Are people going to believe this? I mean, you guys already established a single drop of blood in one guys eye turns him into a bloodthirsty zombie in about 30 seconds. How is a blood transfusion not immediately infected?" and they looked at eacxh other and just went.. "uhhhh FUCK.". They had to call the studio and tell them they needed to shut down production and do rewrites. The studio gave them 48 hours to start filming again.
So they basically said they spent the whole first day pissing and moaning, looking for inspiration, trying to come up with an alternate ending. They said they basically stayed up all of night 2 in a fever just trying to make something that worked. They were super unhappy with the direction they were going until the actors started adlibbing and they were basically taking any suggestions from crew and cast for how each scene should go. They then talked about how well everything just happened to work out, and that they had filmed another ending where the main dude had died and it was just the girls living in the woods making the banner, basically the same ending but a little bleaker.
All in all, the only thing I got from the directors commentary was that these guys tried to make a shitty movie thinking it would be good, and instead fell face first into making a good film thanks to everyone around them. So when 28 weeks later was announced I expected it to be garbage, and it was. I don't know what these guys have done since, maybe they figured it all out, but man, I wouldn't bet #1 on anything these guys do based on that one directors commentary.