My memory is hazy on this. I remember the pig being hung upside down, and this would have been a clear cue that something was going to happen. However, this may have been after it was killed. The reason it was hung was to drain the blood.

Related... the silence of the lambs thing I witnessed first hand. A bunch of sheep taken off to be slaughtered, standing in complete silence. Welsh sheep are never silent, so it spooked me a lot. No visual cues as to their impending doom, yet they all stood there like ghosts.

From Thomas Hardy's book it is clear it is important the pig is kept alive until it has bled to death. Jude messes it up because he can't deal with it.

In butchering on farms, carcasses are hung upside down after the animal is killed.

Pigs are extremely intelligent, on par with dogs at least, and I've seen rankings that place them as high as 4th, behind only apes, dolphins, and elephants.

Sure pigs are intelligent but a pig that's never seen a gun before doesn't know it's going to die.

If there are regular slaughters, smart animals might notice a pattern. "We never saw Fred again after he went to that strange shed. Then we never saw Tom after he went there. Why the hell are they dragging me over there now?!??"

This doesn't mean that any pig could articulate "I'm going to die". That isn't necessary. Nearly every ungulate (maybe not moose?) has a built-in mortal fear ready to surface at any time. New events, or repeated events with bad associations, bring out that fear. It takes a great deal of training to relax that.

The whole concept of thinking about animals dumber than they are makes killing them more acceptable.

It's reading emotions/social cues, not objects. Animals are frighteningly good at that.

Maybe dogs. Not pigs. I've seen it happen several times. The pigs didn't put up a fight or act at all abnormal.