Hanging out with Day al-Mohamed always reminds me to think about these things.
Any novel or longer story (not so much shorts, which have to be tighter) is going to have "extras." Crowd scenes. Characters that show up once never to be seen again. The barrister getting your heroine her coffee. The person behind the desk at the cheap motel.
It's a sad but true fact that the vast majority of our readers, left to their own devices, will populate our world with...straight white people. Because in the English speaking west, that's the default.
So I came up with the term "background diversity." This means adding diverse traits to your extras. Maybe the barrister is black. Or maybe she's wearing a rainbow shirt. Maybe your heroine almost trips over a blind person's white cane while rushing through the subway (I've done that. More than once).
Why is this important?
If your story is set in the real world, you should reflect the demographics of the place where it's set. You're going to find a lot of blacks in New York or Chicago. A lot of south Asians in the English midlands - Leeds and Bradford are both majority colored cities at this point. Or maybe your characters are wandering through a city's gay district, such as DC's Dupont Circle. (Another thing to consider is that if you're in a very white place such as Iceland or parts of Russia, show that too, but show it correctly. There aren't many black people in Iceland. Trust me, I've been there. So any that there are stand out like sore thumbs).
If your story is set in the future, then please, please, please be extra careful to include people of color! I've had fans say that they get actively afraid when they read science fiction that only seems to have white people in it...
In history, again, reflect the real demographics - and do your research. For example, there was a strong influx of African DNA into the European population that we've traced back in time to some time during the Roman period. The Roman trade network resulted in a surge of Africans making their way to Europe, whether as slaves, as traders, or through voluntary migration (almost certainly all three would have happened). So if you're writing in ancient Rome, include black people. We know they were there.
In secondary world fantasy, you can do whatever you like - so why does that almost always mean making everyone white?
Think about it. Even if your main character is white and heterosexual, adding in little touches in the background will show your readers you've done exactly that.