Question: When Robert Neville's family is being airlifted at night, the darkseekers are awake and emerge from their hiding spots. If UV light has been known to kill them, why wasn't his family airlifted during the day?
Answer: They didn't have the time. They were trying to contain the spread but failed and the evacuation had to take place at that time or there was no evacuation at all.
Question: Why were the zombies in a huddle?
Answer: The creatures in this film are not true zombies. They are still humans still alive, but mutated by the virus and in an altered state of mind. They still have their own society and social circles. They often go into a trance like state when dormant and would just be around each other for comfort.
Question: When fixing up and preparing the house defenses, why is Neville so pressed for time? What type of preparing does he do?
Answer: He has to make sure that he's back at his home with plenty of time to lock everything down, sound proof the establishment, and make sure no lights are visible from the outside. His place of residence has to appear like any other place in New York at the time as deserted and with nobody there. If the Night Stalkers saw any evidence at all that that was where he was living, they would attack like they do later in the film. Neville is military trained, so punctuality and having a set schedule to do things is pretty well ingrained into him now. And by giving himself plenty of time and room for error, he can take care of any contingencies that arise and any repairs to make sure everything is ready before the creatures come out.
Neville also uses gallons of chemicals (possibly chlorine bleach) every afternoon to cover his scent in the immediate vicinity of his home. Apparently, the Night Stalkers and their dogs have a keen sense of smell, particularly for uninfected humans.
Question: If Neville knew that the Dark-seekers couldn't get through UV light, how come he didn't rig his home up with UV lights in case of an emergency, or his car for that matter?
Answer: It's likely a question of availability. We see he did in fact rig lights to use as a last resort. We also see that he spends most of his time searching for supplies in the city, so he was likely using whatever he had available.
Answer: This was still very early in the spread and outbreak of this virus. They didn't fully understand what they were dealing with yet, or would have figured out that UV had any effect like that. Also the darkseekers weren't fully out yet, as we see the people infected are still mostly human with just early symptoms. They didn't know what they would create or even knew what it was capable of yet. What you saw were panicked people acting stupid in fear trying to board the chopper.
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