Regarding the whole
fast vs. slow zombies issue, here's my two cents:
If I were to write and direct my own zombie movie, I would establish some sort of pecking order, based on how far gone they are:
1)
RAGERS
These would be of the Zack Snyder 2004
Dawn variety. (Or perhaps a hybrid of the Snyder zombies and Dan O'Bannon's. See my
BRAIN FOOTNOTE.) These are much more aggressive and vicious, and they can run really fast.
However... they won't be able to keep that up indefinitely! The
ragers are the "fresh," newly-minted zombies, and they haven't had a chance to deteriorate significantly. But as a character from Romero's 2007
Diary of the Dead points out, zombies would
not be able to run very well once their Achilles tendons snap (partly due to inevitable decay, and mainly from the exertion of running itself)!
2)
SHAMBLERS
These would be the classic, archetypal zombies as established by George Romero in 1968's
Night of the Living Dead. They would be significantly far gone enough not to be able to run really fast. They would have to overwhelm their prey in larger numbers, as otherwise they would be relatively easy enough to bob and weave and run around. And these
shamblers would be easily elbowed and pushed aside by the "fresher"
ragers who would easily barge their way ahead of them!
3)
SHUFFLERS
An even more advanced state of decay! The
shufflers can do little more than perambulate about aimlessly like sleepwalking mental patients in some old snakepit melodrama. These movements are very reminiscent of the undead in the Italian zombie films directed by Lucio Fulci (
Zombie,
The Gates of Hell,
The Beyond). However, Fulci's zombies seem imbued with weird supernatural abilities, including the ability to materialize and dematerialize at will. They're also really
sneaky, in that they seem harmless enough until you get too close to them, at which point their arm abruptly juts upward and rips out whole chunks of your scalp! Depending upon whether or not the zombie apocalypse is of a supernatural origin, I think the
shufflers would be as (relatively) harmless as they look. (But I wouldn't trust them to babysit an infant child!
)
4)
ROTTERS
The absolute rock-bottom end of the zombie "life" cycle.
Rotters are completely and utterly useless at doing anything other than twitching and crawling about. I'm thinking of a scene from Romero's 2005
Land of the Dead where still-twitching zombie remains are disposed of in what looks like a dumping ground for the decayed, dismembered remains of the undead. From the point of disposal onward, I imagine that one day... eventually... they... would... simply...
stop.
BRAIN FOOTNOTE ALERT!
Another obvious question is: Should zombies eat brains? Or should they be more focused on general flesh-eating? If zombies should be afflicted with this fixation, the only time they would be able to penetrate the cranium would be in their stronger, more aggressive rager state. Also consider that their nerve endings would probably still have some vestigial - albeit fading - life left in them, and as the upper half of a female zombie once memorably put it in Dan O'Bannon's Return of the Living Dead (1985): "It hurts to be dead!" And the endorphins from the brain would (in theory) have a numbing effect on this pain. But I should think that in more advanced states of decomposition, this wouldn't matter so much as their nerves would probably be sufficiently deadened so as to eliminate this preoccupation. (Not to mention the fact that their choppers would not be able to penetrate the human skull in an advanced state of decay. The lower half of the jaw would probably end up dislodged!) From that point onward, they would confine their activity to the consumption of human flesh, which as Romero's 1985 Day of the Dead establishes, is not even a real necessity for sustenance.