Every character – well really every person – has their own unique voice. The challenge for any author is to translate that voice onto the page. In mainstream fiction – you know, the stuff that’s set here and now – almost all of the characters are human and the uniqueness comes from where they grew up and other life circumstances. I’m not going to say that keeping all your human characters’ voices distinct isn’t a challenge. It is. You have to live with them in your head and make sure that they don’t bleed into one another. It’s a challenge. But, if you write science fiction or fantasy there is a high likelihood that you will have at least one character who is not human.
A non-human character provides a whole new level of complication. The challenge is to show the character’s natural disposition to non-human thoughts, while still making them understandable.
A story told from the perspective of a cat is going to be quite different than the same events seen from a human. Cats don’t worry about homework and bills. A mouse is an opportunity for a fresh meal. Even their take on the relationship with humans is different. They certainly don’t consider themselves pets. In fact a cat wouldn’t even think in English. Oops, that would make it a really hard story to sell. So you have to put their thoughts into English and make sure that everything they are referring to is made clear without dropping the feline thought patterns (or the best we can guess at a cat’s thoughts).
Take it a step further, how would an alien describe a human? Aliens would have their own aesthetic based on their own bodies. In this case you would have to know what your alien looks like. If they have long tentacle like appendages, what would they think of our short articulated arms and legs? Perhaps they have six legs and a few more arms, would they be able to understand our ability to sand on just two legs? That’s just appearances. Aliens would think of everything in terms of their home planet. All of their measurements and calculations would start there. Even their morals and religions would be based on that home planet. And on their reproduction cycle. Aliens who deliver a clutch of young at a time aren’t going to be hung up on the life of each child they way that we are. If however they are only fertile twice in their lifetime, child rearing would be a much more important aspect of their lives.
Religion also plays a large role in the thought patterns of a character. Even if they don’t believe in the dominant religion of their culture, they will still be influenced by it. Despite the fact that I don’t believe in Christianity, I still swear by it. That is I use Christian swear words when I’m upset. I count my year based on the Christian holidays (Christmas, Easter, etc) rather than the Jewish or Aztec or Chinese ones. I’m aware of the other holidays, but I remember by world terms of the Christian ones. Why? Because those are the ones that get talked about the most. An Alien would remember their lives in terms of their own holidays. “It was almost Gradfelder Day when my cousin came to visit”.
The key to writing a good Other character, be it animal, alien or magical being, is to make sure you know the background before you start. Know how your Other is like and unlike humans. That will give you the basis on which to add the words in ways that we can understand, but will still seem so very Other.