Generally speaking, your readers will have fewer preconceptions about your characters than they will about your world. When it comes to a world, a reader will assume it’s like the Earth they know, unless it’s explicitly described otherwise.
Yes, you guessed it – another mental shorthand courtesy of our brains. We wouldn’t be able to survive as a species if we spent every day wondering if the laws of physics and the threats outside our window changed without warning. We rely on our air being breathable levels of oxygen, gravity about 1 g, and Newton’s Three Laws of Motion just to get through our day to day lives. We don’t live expecting things to spontaneously catch fire when we glare at them, fish to fall out of the clouds, or pigs to fly. We assume a “standard world”.
Note, this is why people have to be carefully trained for things like spacewalks and underwater archaeology. And why people so easily get in trouble in foreign countries, or the wrong part of town. You assume wherever you happen to be is like where you’ve always been, even if you consciously know otherwise. It gets people killed.
Your readers’ brains will default to “standard world” unless you show them details that prove it’s not. It’s up to you how subtle you want to be. For example, you could have a character let go of his glass of whiskey, and simply never mention a tinkling crash or splashing drink. Or you can explicitly describe it hanging in mid-air, because the station’s in free-fall. Both of these work, but one’s better if your readers want a mental puzzle of deducing how this setting differs from the world they know, and the other’s more kind to someone who just wants an adventurous romp and vacation from reality.
Think about your audience. Pin down in your head what they’re likely to see as strange, and make sure they get a chance to see that strangeness at work before it becomes a major plot point. Otherwise it may look like you’re pulling new setting details out of thin air as plot devices, and no reader’s going to be happy with that.
Of course, what the reader sees as strange and what your characters see as strange can be two different things. After all, your readers are just visiting. Your characters live here. One of the interesting ways to introduce a character is to have them grumping about something they deal with on a regular basis that your readers don’t. Lost handcuff key for a cop. Slime in the upholstery for a paranormal investigator. Throwing up a containment tent over a bed of daffodils because they’ve seeded biting cockroaches.
(That last is in Mirabile, by Janet Kagan. Awesome SF setting.)
Showing what a character’s annoyed by helps show what their priorities are. And it gives the readers an instant connection. Maybe none of us know what it’s like to stake a vampire, but we can all wince in sympathy at someone coughing out nasty, icky dust.
Think about connections, because your reader needs to build a picture in their mind about how your character is and isn’t like the Average Joe (or Jane) they’re used to. Part of what your reader wants will be physical details; rough height, build, hair color, clothing style, etc. We’re visual creatures, we like to know what something looks like.
(And you can’t count on the cover for a good character description. Covers Always Lie.)
But more than the details provided by a glance in the mirror, readers also need to “meet” a character’s profession, and family, if any. Not all at once, maybe not even all in the first book. But those are critical to your readers’ image of what your character is like. If you have a nerdy librarian deadlifting 300 pounds, or a dedicated Roman gladiator whose hobby is knitting… it may make your character unique, but it’s going to require an explanation. Just like your world would if suddenly rainbows set what they touched on fire. Mind, I’m all for weightlifting librarians. It’s just not the cultural image your reader is likely to have.
And cultural images – tropes – are important. All writing leans on shorthands for the complexities of reality; whether that shorthand is brain-based, cultural, or the limits of a particular language. Learn the shorthands. Use them so your readers meet your world and characters fast, so they can dig into the story!