See! I can do what I promise! Although it was close...

I: Aliens, Biology, and Realism: I figured I might as well get this out of the way first. A writer really doesn't need to know chemistry, organic chemistry, and biology in detail unless they want to do the really weird environment aliens. In other words, unless you start putting your aliens on moons with no atmosphere, not-quite jovians with H₂ based atmospheres and many many gravities of, err, gravity, or other weirdnesses, you don't need to work out the biology.

Why?

Simple. We live on a 1G planet with an oxygen-nitrogen atmosphere. Life in various forms is all around us. There is significant evidence that life will evolve similarly on other planets with similar atmospheric and gravitational conditions.

In other words: Just make it something that can exist in your backyard and you can't go far wrong.

Of course, a certain basic knowledge is required -- insects don't scale up very well (they don't breathe through lungs and the way they breathe would not work at larger scales), there are size limits on flight, etc. However, using a mammal as a base form, it's not hard to make a viable sentient without an immense knowledge of biology. You just have to keep a few questions in mind that do need answers:

1. How did said creature evolve? In other words, if said sentient alien has six limbs, they likely all other life forms on the planet will have six limbs (like most mammalian/reptilian/amphibian life on earth has four limbs).
2. What niche did they have before they developed a brain? Were they herbivores? Carnivores? Omnivores? What environment did they live in?
3. Why did they develop a brain? Current wisdom holds that carnivores and omnivores are more likely to develop intelligence because they need to think more to capture prey and eat it. As to humanity, my personal belief is that we lived at the border of two environments (plains/forest) and it was easier to develop a brain and adapt both environments to us rather than adapt our own forms to work efficiently in both environments.

II: Alien Personalities: This is the big one, and the one that is both the hardest and the most important. Note, however, that it really doesn't require a knowledge of science, just some thought experiments and background to give some basics.

Aliens in SF really come in three types -- humans in alien suits, animals with brains, and the aliens we simply can't understand. Let's look at each of these in detail.

Humans in Alien Suits: This is the alien from the 50s, and from Star Trek. Basically take a human, give them extra limbs or bumps on their forehead or fur, but have them act exactly like humans. Easiest for casting directors, simplest to write, least effective, and most likely get you laughed at by serious SF fans.

Of course, there are ways around this -- aliens genetically engineered from humans, or people who TF into furries, would be like this. So, largely, would animals uplifted into intelligence because we'd make them in our own image. In other words, this method can be used, just make sure there's a good reason why they're humans in alien suits.

Animals with Brains: This is the most common 'alien' floating around today. In essence, the physical form is based on a class of life on earth, and their personality is strongly influenced by that life form's characteristics. It is a compromise to make an alien seem both alien, and yet understandable to the reader.

As an example, in C. J. Cherryh's The Pride of Chanur novel (and sequels), the primary point of view race . The Hani that are the main characters in this novel are lion-people, plain and simple. However, they feel alien because the author has taken feline characteristics (prides, females being the "hunters", etc) and made them a dominant factor in their characters. Like humans have a drive to have their social group be dominant, the Hani have drives to protect the males, and to boost their individual pride. In other words, they simply have the characteristics of felines.

Another example, are the centaurs in my story Being a Patrol Sentient. The centaurs are both herbivores and herd creatures; thus, the POV character, a centaur, needs close contact with friends -- with his herd -- for his mental stability. A factor that makes him seem more alien, and yet it is comprehensible enough as most of us have some knowledge of herd instincts amongst herbivores that it seems realistic.

The trick here is to not make it too obvious. I completely missed the Hani feline derivation for years until it finally hit me. C. J. Cherryh made it such a matter of fact property of Hani culture, and did not describe it in a familiar fashion compared to terrestrial feline sociology, that it was not immediately obvious to me.

Aliens We Can't Understand: These are probably the most realistic, and yet the most rarely used. Basically they do stuff for reasons we simply don't understand. Or, they perform actions for no apparent rhyme or reasons. And this is why they really don't work in fiction. If we can't understand the motivation, we deeply distrust and fear the alien because, to us, they are insane. Psychopaths. And, in terms of story plot, the fact that they have no rhyme or reason makes it very hard for them to be a major part of the plot as the reader will simply perceive them as the author "breaking the rules" so that the reader cannot make an educated guess to try and solve the problem the characters in the novel face. It's kind of like reading a story where, without any cause or rhyme or reason, the villain suddenly gives himself up to the police and fixes everything. It isn't satisfying.

Aliens like this can work in a minor role, as something mentioned in the background. Or they could be used as a kind of bogeyman to terrify the POV characters. But, they cannot be instrumental to the plot for the reason that they cannot be understood.

Use them with caution.

III: Alien Appearance: I've left this to last because, ultimately, it's the least important (to me anyway). An alien has a personality, motivations, goals, which drive the story. And if he also happens to have 8 limbs and two heads, well that's nice but not as important as his/her/its character.

Effective aliens generally tend to be either really alien in appearance and shape, or strongly based on a non-sentient earth animal. Alien in appearance aliens tend to have extra limbs, brains and heads in seemingly odd locations, non-symmetric forms, etc. Animal-derived aliens tend to be cat-people, horse-people, etc. Of course, these can be very effective -- look at Larry Niven's Kzinti, or Poul Anderson's Merseians (cat-people and lizard-people, respectively). There are some differences in physical appearance (Kzinti have ratlike tails and furless ears, Merseians are warm blooded) but by and large they're cats and lizards.

The most important thing to keep in mind when determining the physical appearance, is having in mind why they appear that way. For creatures which evolved naturally, the 'why' is the simple Darwinian logic of survival and natural selection; if the creature was constructed by another race, the 'why' is the purpose for which that race built the creature. Either way, every piece of the creature must make sense in terms of the 'why' of its existence. You should at least have an idea before you start writing so that you know. And, happily, this knowledge will almost always give you a better alien and a better story.

IV Conclusions: Basically, as long as you stick with oxygen-breathing mammalians, make sure there's a reason for their appearance, have some idea as to how and why they evolved to sentience, and give them some instinctual characteristics of similar earth animals, you can't go far wrong. Kind of a long sentence, but true. Good luck!

Next time: Space Ships!