Building a more "realistic" Sci-fi Universe with aliens

[Simon_Jester]

The real problem is that as a rule, the author has to know the reason things happen, even if the reader doesn't. If the reader can't figure out how the aliens operate that's fine, but the author has to actually portray their operations.

Trying to write the actions of beings you lack the knowledge and intellect to understand is how you end up with an Archinist story.

[Korto]

I start from first principles. I figure out what I want their planet to be like. Then I figure out their basic biochemistry (which in most cases will be carbon based but there are variations that can affect other things). Then I figure out what sorts of things might evolve on a given planet and how they might be adapted to live there.

Which is pretty much the opposite of how I do it, which is to think of an alien concept I like first, and then try to work out how such a thing could come to be--what kind of natural niche they filled, and therefore what their environment would have been like. This then often always causes changes to the original concept through feedback, more depth and things I hadn't realised until I'd started thinking about how they'd work.

I had at one point been thinking about an octopus-like creature, well, more like an octopus crossed with a giant amoeba. It was capable of forming pseudopods to move around and manipulate objects. The species was more advanced than us.
I decided, as technology seemed difficult for an aquatic species, that they'd come out of the sea long ago. That then necessitated that their skin had toughened up, which made me think that maybe they had lost the ability to just freely form pseudopods, and they now had a few permanent ones. Lets assume they used colour and movement displays when in the water to attract mates, an ability which has now been lessened; really they should probably have abandoned that method and changed to something more suitable for land, but lets assume they kept that because we can then suppose that there was a second, related species and they stayed in the water. They thing is, staying in the water meant they stayed primitive, but kept their full ability at colour display and movement, making them now incredibly sexually beautiful to the land dwellers.
Then they would interbreed and become basically one species, right? Not if they're not similar enough for that, very few births and all sterile, for instance, which would possibly lead to a mental/cultural revulsion to interspecies sex. It would be taboo. So they would be incredibly beautiful, graceful, and wrong, like say Slanesh daemonettes.
Lets also assume these land-dwellers aren't terribly nice, and pornography is just as popular with them as us.

I don't do much thinking about polarised light vision or things like that. Wouldn't know where to start.