Most writers are extremely observant - watching mannerisms, eavesdropping conversations, aware of the emotional ebb and tide of situations - and filing these away for future references when they might need them in a story. But don't forget to add the elements of place (Setting) that will increase your reader's enjoyment and understanding of the characters and their situations.
Whether it be the quiet hush of a deep redwood forest, or the noise and constant activity of a city street, show us your characters' surroundings, with enough detail so that we might actually picture them there.
Practice taking pictures in your mind as you go through your daily life, or as you travel, or even as you watch movies or television. File those things away for later use, so that you might be able to recall the wide open vistas of a high desert, or the dusty chalkboards of a classroom, or the hulks of discarded vehicles in the tall weeds of your neighbor's yard.
I don't care how good your story is. A well-told setting, described so that we can picture it in our minds as if we were there, is a requirement, in my opinion.
Can you think of any great settings you remember from stories or novels you've read? How much did it lend to your enjoyment of the story?