Believability

HHowever alien our characters are, our story has to be believable.  Most of the time, although we're writing science fiction which might involve aliens, we're writing to enlighten ourselves.  We write to make sense of humans, our culture and beliefs, and the best books are the ones that get us thinking about who we are, what our values are, and what our cultures are doing.  The best SF allows us to understand ourselves better.  And if our story is not believable its lessons will most likely be lost.

Believability in a story comes from many things.  If your action is set on-planet, it comes from a richly drawn world.  The flora and fauna is properly thought out, and obeys the laws of physics and evolution.  If there's more than one sun in the sky the shadows will be complex, and would confuse a human newly on-planet.  The sky might be a different shade of blue. The differences might be subtle, but they'll be there.  

The political and cultural systems of the world need to be logical within the premise of the story, fit together, and be consistent.  Which can often be a challenge when that culture and its politics are radically different from ours and something we're struggling to understand.

If we're setting our story on a starship, we need to make the operating procedures and the way the ship travels believable.  One of my favourite books for this is C J Cherryh's The Pride of Chanur.  Cherryh has managed to portray the everyday ordinariness of a trading captain who is constantly travelling in and out of jump.  She shows the physical toll this constant travel has on the crew in little touches, like Pyanfar shedding too much hair (pelt, not head hair) when she showers.  She's worked out the ship's operating procedures and the way the crew works together in dazzling detail.  Even now I still marvel at its complexity when I go back and re-read it.

If we're going to mind-wipe citizens we have to set up a culture that will allow that monstrous act.  The chances are we'll be dealing with a brutal totalitarian regime that rules by fear.  If we're going to control and sell the air, or water, of our world, we'll have to set up some disaster that makes these resources scarce, and put someone into a position of power where they can control them.

Culture, politics, flora and fauna.  All these have to work together, and be worked out in detail, if we're to make our stories believable.