A character is introduced that is out of this world. Maybe he’s a demon or an alien or even a robot. However, as the characters start getting to know him, they realize that this new guy, who comes from a world completely separate of theirs, is actually… just like them. He’s “one of the guys,” so to speak, and no quirk of the personality or strange appearance really changes this.
Why this can be bad: This doesn’t have to be bad. In fact, it can actually work very well. It just depends on the kind of story you’re trying to tell. Star Trek is a great example. Star Trek has always been about inclusiveness and acceptance. Started at a time when the Civil Rights movement was underway, it made sense that its humans were human and its aliens were human, too. Whether Gene Roddenberry meant for this, it doesn’t matter. The point is that the humanizing of its aliens emphasized its ideals and themes. The aliens of the show are often human-like in appearance and personality with just a few tweaks. And that worked. However, what if one of Star Trek‘s themes hadn’t been inclusiveness? What if it wanted to focus instead on world-building and the vast differences in sentience? If that had been the case, the show wouldn’t have made sense. After all, these aliens are so incredibly human, in culture and commerce and clothing and a host of other things. With the theme erased, the audience would be left wondering why these creatures — which were born in different worlds, solar systems, universes than us — were somehow nearly identical to us.
How you can fix it: First, you need to assess what your story is about thematically. What message, conscious or otherwise, do you want to push? What atmosphere do you want surrounding your non-human creatures? In Death Defiant, I analyzed what I wanted my different subspecies to be like. It would make sense for half-demons to be human-like. After all, they’re raised on Earth and have the genes, so why not? Even demons are pretty human-like in most respects, though they have a tendency to be aggressive. In that sense, I mirrored the way first-generation Star Trek created its aliens. Angels, on the other hand, had to be vastly different. They had to be the kind of creatures that could be worshiped by early humans and seen as being superior to humans. Since I wanted to drive that home and since angels and humans look identical, it was essential I made them different culturally and physiologically.
So what do you do if you’re in a similar situation with your non-humans? A lot of world-building. You essentially need to create a new culture in order to show how these creatures are not human. In DD, the angels have an evolutionarily ingrained reverence of selflessness because of their grief chain (meaning that if one angel dies, other angels close to that one could die of grief, creating a chain of death). In their stoic forms, they are incredibly dense and heavy because they’re carrying a much larger form within their body that needs to be expanded. They have a trade- and karma-based economy. They are asexual and agender, and their way of creating new angels is based entirely on manipulating their biological magic and energies. They may look human and almost act human, but they are definitively not human. These are the kinds of details you need to think of when creating your own non-humans. They need to have their own worlds, and their existence and development must mirror the way their worlds developed. Otherwise, they’ll come off as nothing but green-skinned humans.
Bottom Line: If you decide your non-humans need to be distinctly non-human, develop their species’ history, physiology, and culture in a way to set them apart.