One way in which SF writers who also create visual art have a huge advantage over everyone else is in the all-important area of world-building. If you're creating an alien world, or a futuristic setting, from scratch, the ability to visualize it beforehand is crucial.
"All along, I've had a visual imagination," says Rucker. "For me writing is a little like dreaming while I'm awake. That is, I see the scene in my mind's eye before I write it. Sometimes I'll nurse an image of a place or a situation for quite some time before I write about it, in fact I sometimes write a book simply to be able to mentally visit certain locales that I've dreamed up. I pretty much can't write a novel unless I have an image of a fabulous place where I want to go."
And when Rucker gets to write about those imaginary locales he's visualized, they become more real to him, and thus more compelling. It's sort of a feedback loop. "And painting is a way to layer on more details."
As for Kowal, she says working with puppets definitely helps her craft a scene or a locale in her mind. "The world-building is actually one of the key things that pulls me to both puppet building and SF and fantasy. Both forms are essentially the theater of the possible." At the start of a new story or a new puppet show, at first anything is possible — including defying the laws of physics.
The puppetry training also helps Kowal to visualize a new world, because her training has taught her to start a project by defining the parameters. "Since I'm creating a world from scratch, I need to make certain that the visual language I use is consistent and that the world has an internal logic. It's the same with writing."
For me, a story will evolve from a drawing (or an imagined visual) as often as a drawing comes out of a story. I'm writing about a space station. The more I pre-visualize, or draw, the interior and exterior of that station the more "real" the station becomes for me and the characters. The better I understand how certain areas look, smell, function, flow, etc., the better I can write about them. Any writer can do this (and for the most part does and should), but having trained and practiced the visualized representation of imagination, perhaps a visual artist who also writes can have an advantage. Of course, there is more to writing than describing visual experience, so it's not necessarily a "great" advantage. The processes in art/creativity tend to be similar.
Niffenegger says world-building is less of an issue for her, because she tends to set her stories in existing places. "The world is built." Where visualizing the story comes in, for her, is figuring out the best real-world location to set off something that happens in a scene.
Keeping it concrete
And perhaps the most important benefit of having access to another art form is that it helps you make the story more concrete. Kowal says that being concrete helps you think about how one little change alters everything else in the world. When she teaches about stage adaptations, she always says, "If you change one thing, you have to look at how it changes everything." And that comes back to writing SF: "Writing about the future is basically taking our world and making a change, which affects everything."
It's all very well to have crazy ideas and whiz-banging plot devices, says Rucker. But in the end, "everything has to be visual. I think I learned that from Robert Sheckley and Jorge Luis Borges. Ideas are important, but what you want in a novel is an objective correlative for the idea."
So instead of going on and on about your crazy ideas, you want to show the reader "some weird little physical device. Imagine, say, a wriggly green horseshoe with antennae on it, call it a jinkerand when you point your jinker at some object, the target object becomes weightless and the size of a matchbox and you can carry it off in your pocket. Maybe the jinker talks to you telepathically, maybe pairs of jinkers like to get together and mate, and while they're doing it, all the objects in your house are floating around and changing size. That's all much more interesting than talking about spatial metrics and gravity tensors!"