I remember the scene where Sebastian asks the replicants to demonstrate something, and then Pris puts her hand into a boiling water to take an egg without feeling pain and at another point Batty breaks a wall with his head. This means that replicants don't feel pain.
Now my question is: If this character:
Deckard
is a replicant, why does he feel pain when Batty breaks his fingers?
Until the events of the film, the replicant Rachel believed herself to be human:
DECKARD: She doesn't know.
TYRELL: She's beginning to suspect, I think.
DECKARD: Suspect? How can it not know what it is?
That would include having a normal pain response. If she could put her hand into a pot of boiling water and not react, it would be a rather large clue that she was not a typical human being.
Therefore, some replicants are designed with a pain response. If Deckard is in fact a replicant, this must apply to him.
Incidentally, it appears that the other replicants do feel pain. In a later scene when Roy pushes a nail through his hand, he looks distinctly uncomfortable -- indeed, he's doing it in the first place to experience the sensation of discomfort. However they appear to be capable of ignoring pain when they so wish.
Your basic premise is not valid; replicants do feel pain.
Near the end of the film Roy Batty was trying to stave off his impending death. He pushed nails through his palm to provide a strong stimulus; Batty was visibly in agony while doing this. As another example, Pris' heel-drumming and demented screaming when wounded by Deckard's gunfire were evidence that she experienced pain.
In humans and their meatspace analogues, pain response is a symptom of damage in normal situations. So if a replicant's skin is durable enough to not be horribly burned by a plunge into boiling water then there should likewise be no pain response. Pris' hand looked undamaged by the hot water. By contrast, Deckard's fingers were broken or at least dislocated with badly torn ligaments. There was damage, therefore there was pain.
that hurt.
There actually was a 60 Minutes or some other news show decades ago about a family (I think with a lot of cousin marriage) where many of them due to a genetic condition did not feel pain. And not surprisingly, they had all sorts of broken bones. (I think leprosy also leads to nerve damage that can in part be responsible for the appearance of the affected although I think also there is a direct effect also.)
I would guess such people would be dangerous for a normal kid to play with.
Anyway, pain serves a purpose. I would suggest that when Pris stuck her hands in boiling water to fetch the egg, she was able to turn off pain -- also her skin was more resilient. Leon also plunges his hand into liquid nitrogen, I think.
However, Roy Batty during the fight with Deckard says, That hurt! at one point.
Maybe they can't turn off pain but have both higher pain thresholds, and to go along with this, tougher bodies. This makes sense: no point in experiencing pain if the body is not at risk of damage.
