Not to suggest that we're already great when picking our words. If you're like me, during particularly rushed or stressful times, an entire sentence can fly out of my mouth and not make any sense at all. It happens. We're human.
So how important are words in writing?
When we create a scene, a conversation or show what characters are doing, it's important that the best words are selected for creating the mood. For example, in And the Devil Will Laugh, I had help preparing the back cover because I wanted to be certain that I chose the best words that summed up my book. I wanted to show the intensity, the fierceness of the characters, to create a sense of what to expect....
...he and his loyal foot soldiers will muscle in and tear off a big chunk of it for themselves.
This, of course, creates a pretty strong impression of what kind of characters can be found within the pages of the book. If I had chosen instead to simply say that Jorge planned to 'move in and with the help of his associates, would take over' it simply wouldn't have the same impact.
Also, keep in mind that each character has their own kind of dialogue. For example, Jorge Hernandez, the protagonist in my last few books, is known for often saying "I don't got time." When he says this, it's usually a sign he's aggravated and is about 5 seconds from rushing out the door and he wants a situation resolved now. Each of my characters has their own, unique dialogue patterns and expressions.
With dialogue, its also important to remember that it can vary according to a character's education, age, ethnicity, experiences, and even attitudes.
Words do matter. Pick yours carefully.