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 I create a scene, a conversation, or show what characters are doing, I am careful about picking words that establish the mood. For example, in And the Devil Will Laugh, I had help preparing the back cover because I wanted to be sure that I chose the best words that summed up my book. I wanted to show the intensity, the fierceness of the characters, and 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 creates a pretty strong impression of the kind of characters in my book. If I had chosen instead to 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 a unique 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 unique dialogue patterns and expressions.
With dialogue, it's 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.