They say you are what you eat. We don't know who they are, but storytellers must take their advice to heart because food and drink are often important aspects of characterization. In The Island of Dr. Moreau, it's not just about what characters eat but also about what they aren't allowed to consume. The Law states the Beast Folk aren't allowed to eat "Flesh or Fish" (12.18), and they can't "suck up [d]rink" like a dog (12.18).
The Law is used by Moreau to keep the animal side of the Beast Folk suppressed. So, when Leopard Man and Hyena-Swine kill rabbits, they are characterized carnivorous and bloodthirsty, even if it is in their nature to be so.
Alcohol also helps us understand character. When characters drink in Dr. Moreau, they become beastly in nature. Captain Davis is a royal jerk to M'ling when he's drunk, and that man is always drunk. Montgomery is very tender to M'ling but "[will] ill-treat it, especially after he had been at the whiskey, kicking it, beating it, pelting it with stones or lighted fuses" (15.12). Even M'ling, a generally gentle fellow, comes to a violent end, his cold, dead fingers still clutching "the upper part of the smashed brandy-bottle" (19.30)
So, drinking alcohol leads to unleashing the animal inside the character. Whether that character is a person or a Beast Folk doesn't matter. The differences between man and animal break down. Just go to the happy hour at your local dive bar...
Horror movies typically characterize with an "us versus them" mentality. For example, it's humans against zombies, or humans as team A and vampires and werewolves as team B, or worse—the dreaded horror of P.E. class—shirts v. skins.
At first, it seems Wells uses physical appearance for similar characterization in Dr. Moreau. We have the human characters of Prendick and Montgomery on one side, and the Beast Folk on the other.
Early in the novel, Prendick sees M'ling's animal eyes glow in the moonlight, and the sight "struck through all [his] adult thoughts and feelings, and for a moment the forgotten horrors of childhood came back to [his] mind" (4.20). Clearly, Prendick doesn't want to be friends with the guy based on how he looks. Normally, we'd call him shallow, but in this case, yeah, eye shine can be pretty freaky.
But then, just when we get comfortable with this, Wells switches things up. Later, Prendick sees a bovine creature at work and "[finds himself] trying hard to recall how he differed from some really human yokel trudging home from his mechanical labours" (15.13). After all, even though the Beast Folk are physically different from full-blown humans, they do share some human traits. The differences get blurred.
We go from dividing up character based on differences of appearance, to uniting physical appearances based on similarities of character. Go team(s).
Like "Physical Appearances" above, social status starts out nice and neat but ends up in some murky characterization.
When the novel starts, it's simple: the Beast Folk are the lower class (poor) and humans are the upper class (rich). The Beast Folk wear rags, live in some seriously nasty huts, and basically are Moreau's slaves, doing whatever he needs. The humans live in the enclosure and carry guns and whips—just in case those Beast Folk get a little uppity.
However, Moreau notes, there "is a kind of upwards striving in them," perhaps a desire to become more human (14.47). This desire to be more human, not just physically but emotionally as well, makes sense as the more human Beast Folk are treated better. M'ling, the most human of the Beast Folk, lives in a kennel near the enclosure and travels the world with Montgomery. Being more human is like moving up the social ladder.
But that's too easy, isn't it? As we read the novel, we'll begin to notice Wells doing what he does best, breaking down this characterization. It starts with Satyr and Ape Man openly questioning Prendick's upper status on the island despite the fact that he's a full-fledged, genuine human being (16.8). It ends with the enclosure burning to the ground, and Prendick moving in with the Beast Folk to survive. Whatever social distinction separated Prendick from the Beast Folk breaks down at this point.