For me – and I’m guessing this might be true for other writers too – the process of redrafting an sf story is in part one of removing narrative training wheels. By narrative training wheels I mean story elements that the story is better off without, but that make the story easier to write.
For example: I recently finished a short story set in a future in which people exist as minds uploaded into computers. In my first draft, the story started with the main character’s space ship growing a new body for him and downloading his mind into it, so that he would spend most of the story as something like a present-day human. In the second draft, I did away with the body, but had him living in a simulated environment that gave him a virtual body and virtual props to interact with. Only in the third draft did I do away with the simulated environment and tell the story from the point-of-view of a truly disembodied mind. This made the story more focused and interesting, but I’m not sure I’d have been able to write that story without first writing the versions with the ‘training wheels’ of a more mundane environment.
Another example: in the first draft of Belt Three, all spaceships had TV-style artificial gravity. Each ship had a ‘down’ at right-angles to the direction of travel, and when you came in the airlock you’d fall to the floor when the gravity was turned on. When I rewrote it, I decided to go for a more hard-sf setting where ships have parts that rotate to create gravity and parts where you’re weightless. This improved the feel of the book immensely (and gave me some ideas for the setting, e.g. the relationship between social class and one’s home gravity) – but when I was writing the first draft, the training wheels of artificial gravity helped me focus on the character drama without worrying about how people were behaving in microgravity.
One final example, to demonstrate that this isn’t just applicable to science fiction: I recently finished a story about a character on a journey (original, I know). My first draft gave him a companion, and a lot of the story consisted of the two of them interacting. The final draft cut the companion entirely, but writing the interactions between the characters had helped me work out the story was really about.
So the advice I’d derive from this is:
1. Don’t be afraid to use narrative training wheels
You know rubber forehead aliens aren’t realistic, but when the aliens are interacting with one another you can’t help imagining them as human, with human body language and modern English figures of speech. So go for it: make them humanoid in the first draft while you get the drama worked out. Proud refusal to use these narrative training wheels could lead you staring at the blank page, or wasting your time devising alien biology when the important thing is to get the story written.
(Sometimes, for some writers and some stories, establishing the details of the alien biology is genuinely the best way to start. But sometimes it isn’t. Be honest.)
This is really a special case of “the first draft doesn’t have to be perfect, it just has to be written.”
2. But revise them out of your second draft
Once you’ve got the essentials of the story down, that’s the time to remove the training wheels for the next version. Don’t leave them in just because they worked for the first draft. Even if they’ve led to a scene that you’re proud of (I quite liked my description of my posthuman protagonist being downloaded into his newly-grown body), the story will be better. Learn to recognise what’s a story element that genuinely needs to be kept, and what’s a training wheel that your story has outgrown.