Realism. It varies wildly. The most realistic zombies are usually the ones that come from the least realistic origin.

Magic zombies in particular tend to be static. They get raised, and whatever condition they're in when raised, they're animated by the magic, so it doesn't matter what that condition is. They don't need muscles and tendons at all, if the writer doesn't want to worry about it. But magic also allows the zombie to be fully functional on raising if the writer wants, so they can look alive, and move as well as a living person, and it isn't unreasonable.

But then you get into the other types.

Most of the parasitic or symbiotic zombies tend to not rot at all. The only degradation occurs through injury, which can heal since the zombie isn't actually undead in every case. When that version of zombies are dead bodies being inhabited, they tend to decay, and the inhabitant creature looks for a new one, which is pretty realistic.

Chemical zombies tend to hew close to realism as well. They tend to be living creatures that act like zombies, rather than dead things brought back to life, and the few examples in fiction tend to have the zombies degrade over time until they're less effective, which is realistic enough.

But, man, the pathogen zombies get crazy.

The walking dead is probably the best example of both realistic and unrealistic pathogen zombies I've seen. There's a delay between when an infected living person dies and, then a a slight delay before they rise, which is how a pathogen should work. You wouldn't have instantaneous action on death because it takes time for cellular activity to stop, and the brain to fully shut down where the pathogen could take over the body.

But, the zombies rot. The rot is slowed compared to a normal body, and a lot of normal decomposition is bypassed entirely. There's no bloating in TWD zombies, and very little skin slippage. There's reduced color changes as well. This is largely hand waved the same way magic zombies are; "it's the pathogen" is the reason for the bodies not decomposing in the usual way. I've never seen a zombie book or movie explain how the various pathogens prevent decomp stages, but it's the default that the early stages where things fall apart just don't happen.

The reason for that trope is obvious; if they rotted normal, they'd be down to a bunch of barely connected bones that couldn't move at all way too fast (a matter of days in some cases of real decomp) to be interesting. So we all kinda know that there's some hand waving going on, and suspend disbelief regarding that part.

But, again working with TWD, the progress of the zombies breaking down is where things get unrealistic anyway. They do a great job with the anatomy usually. The prosthetics and makeup do a great job at showing what would be under the places where skin is gone, or muscle is gone.

But they rarely get what would and wouldn't be able to move right. When you see a zombie I TWD where the rub cage amd sternum are visible from the front, that means the pectoral muscles are gone. So, how is the zombie reaching forward? And that's my usual example from TWD because they have done that exact thing so many times. You'll also see zombies where the jaw bones are visible, almost back to the joint, and they're still biting with full force. Hands that lack visible tendons grasping with full strength.

The anatomy as shown is amazing looking, but they make the mistake of not making function change when the zombie needs to be a threat. When it comes down to zombies not matching their degree of degradation, TWD is the worst offender because they rely so much on the visuals for the terror. They want heavy decomposition effects visually, but just ignore function entirely.

Compare that the the resident evil stuff. Different kind of pathogen, and the zombies don't usually look realistic at all, but that allows them to perform how ever the script and director want. So, despite having poor anatomical depictions from injuries and degradation, the degree of movement available is still realistic enough to not break immersion.

Which, that's one reason you run into realism discussion around movies and shows, but not books or comics. Without seeing movement, you don't think about it. TWD comics, you don't have to see that an arm isn't moving in a believable way because it isn't moving. In books, you only see what the author describes, and fill in the rest with your imagination, which just "fixes" any unrealistic description.

But, there's still another level of realism. Why should there be a zombie at all? How/why is it going on?

That's where the usual pathological zombies win out. As The Last of Us so effectively shows, there are most definitely microorganisms that change the behavior of a living thing, to some degree or another, as part of its life cycle. Toxoplasmosis, cordyceps, those are the two best known, but there are other real world organisms that co-opt a host and make it do things that it wouldn't normally do, with the express goal of reproduction of the microorganisms.

The ones based off of those principles tend to be realistic enough in how they're executed that it's terrifying, because it's only the scale that is questionable, and it's that *maybe" that generates the fear.

The other kind, like resident evil and TWD, where the pathogen is human created, but gets loose (which isn't ever confirmed in TWD, but is the most likely origin based on the scenes in the CDC, imo) exploit a different kind of "maybe" to drive horror. We know that people have created biological warfare strains, and that they've been used. We know that people are able to engineer some really amazing things into microorganisms. So what if someone screwed up?

None of the pathogen zombie examples I'm familiar with goes into true detail of what changed in the pathogen, which is the same kind of hand waving that magic zombies get, and that book zombies get. The creator of the media says "they changed it so that this happened, using insert technobabble here, and now we're zombies", and the audience accepts it because we just don't know the technology behind editing the genetics of a virus. It's trilithium crystals and midiclorians for zombies, only more realistic because gene editing does exist.

At this point, I think I covered the bases well enough to cover the original goal, and to go further would start getting into individual movies and episodes, and require more linking than I've got time for to get a really useful reference going, so I'm going to leave this where it is. However, I fucking love zombie fiction, so I'm down for non essay chatting lol