Well, I'm a fantasy writer, so setting is important*. In fact, the characters and plot came from the setting idea. Basically, the world is such that seasons last for years (19 to be exact) and change overnight, and animals (and people) change form depending on what season they were conceived in.
The protagonist grew up in a town made entirely of 'Middle People' -- normal humans, all born in autumn and spring. Her culture is a loose series of independent but trading cities on a mountainous coast. They practice strict birth control to ensure no babies are born in winter or summer. They also are matrilineal but patriarchal -- men hold office and manage trade, but inheritance flows from uncle to sister's-son.
I picture the countryside in spring looking much like it does here in New York. In summer, the trees stay the same, but the annual plants grow almost tropical, and the weather is a bit warmer than upstate New York.
Also planning on showing the summer camps of the Summer People and Winter People -- both of which are semi-migratory, since they need warm or cool conditions to be comfortable. So a trip up into the mountains, and back down to near the sea. And some cultural differences -- all the Winter people will be adults in their 20s and 30s, and the summer people will be all oldsters (60s and 70s) or infants.
* Well, it should be important for every writer, but less is familiar for your readers. Unless you're writing urban fantasy, I guess.