Revealing mistake: When Venom violently opens the large portal of the church, he barely takes one step but the doors close basically through him. (01:08:15)
Revealing mistake: The back of Cletus Kasady's head appear quite different between the semi-POV shot in the car (stunt driver) and the next shot when Woody Harrelson steps out of it and gets to the gas station where he will 'hack the internets'. (00:49:40)
Revealing mistake: Kasady squishes a spider that is walking on the postcard. The splattered blotch, strictly divided in red and black, is disproportionately large for the size of the insect. (00:25:35)
Revealing mistake: In a random act of road rage, Carnage sticks out a couple of tentacles, effortlessly lifts up a truck that was driving by, and tosses it off the bridge. Great, but no matter how strong Carnage and each tentacle of his is, it can't nullify physics; it lifts that vehicle stopping entirely its momentum, and Carnage's car does not even remotely budge for the added weight. (00:45:25)
Revealing mistake: If you look at the drawing in front of Eddie when he glances down at it after Venom typed in the name of the beach, it's only a crude draft of the one in close-up (it's missing the wave lines below the cliff outline, for instance). (00:11:20)
Revealing mistake: Shriek's scream is strong enough to cause the explosion of a helicopter at great distance, but the frame of the windshield is unscathed. (00:54:15)
Answer: The host and symbiote merge fully. So the symbiote can totally disappear into the host and the host can totally disappear into the symbiote. They can also split again, or partially, at will. It just depends on who gets to be the active version at that time.
lionhead
I am not up to speed with recent Marvel canon, but in the comics it's never been that way? The symbiote can surely slink inside the host (especially Carnage in Kasady's blood), but the humans can't turn into shapeless goo. Comics aside, that sequence from the movie is mind-boggling; I can sorta explain it thinking the symbiote just tore Kasady's torso in half and then reattached it instantly (in other parts of the movie Eddie gets basically stabbed with what would be lethal wounds).
Sammo ★
Actually, in the comics it's long been established that Carnage's healing factor is Deadpool-levels of broken. There are numerous moments where Carnage is impaled, crushed, decapitated, has his neck twisted, even grenades blowing up in his jaws and straight up nailed by military missiles... AND HE'S JUST FINE AND WALKS IT OFF LIKE NOTHING HAPPENED. He absolutely could casually tear himself open with no drawback whatsoever.
"Could" tear himself open, does he usually?"Can turn into shapeless goo", has he? One thing is to regenerate the torso, another thing is to manipulate your body parting before the bullets reach you.I don't really see that from the example posted, but my curiosity aside, given we're talking about the movie anyway, I really don't see Kasady depicted as a shapeshifter, and him and the symbiote in this movie are entirely separated at the end (pending a sequel of course).
Sammo ★
Also, for Carnage specifically, the human absolutely can turn into shapeless goo. Makes sense, actually, given that the symbiote canonically merged into Kasady's own cells and microscopic DNA, something even Venom and its hosts can't replicate https://2.bp.blogspot.com/9DjIg5e1HwLrwx-lhLjXxlUqzici7xajVTQZMhEHW8a0X9BqdRFE4U6eaBuPKXJgb8zSxkTytpvh=s1600 so the "symbiote-opening-up-the-host-body-with-holes" being a Carnage specific thing isn't surprising at all, in fact, given it's the same body.