As a writer, I have learned value of properly used words. You can state something with whatever tone you like based on the words you choose. A single meaning can be linked to multiple words, each with it's own nuance and level of severity.
For example, is my character mildly annoyed, angry, or is he enraged?
Each emotion varies by degrees and, when used properly, can paint a picture of your character's emotions and personality.
Each character can be seen with important points, that are varying distances from each other, depending on how each relates. These points of personality make up the characters values and morals. What connects them are the emotions and reactions in between.
Picture a connect-the-dots style puzzle, however instead of lines, a writer is using words. Misused words will weaken the strength and believability a character possesses. A generally jovial and pleasant character would not have an angry outburst at something for no reason, unless it plays up to one of those stronger personality points. Consistency is important, even if the character is irrational in their behaviors. They should not suddenly become rational without explanation. I stress the explanation part, because characters grow and change as real people do, but the overall shape of that character should reflect this. While that character may not stay jovial the entire way through, we should have some inkling as to why.
Another misconception I have discovered is that writers have the dictionary memorized. Maybe some do, but I certainly do not. Sure, reading and writing gives us a wider vocabulary to choose from, but we are not born with these words in our heads. We had to go out and learn them, just the same as the next potential writer.
In real life, no one is perfect and we do not always make the best word choices in daily conversation. As writer's though, time is on our side to some degree. We have the chance to rethink and reword, and explore other options. Just as long as we keep the flow of dialogue natural.
Words can be worth their weight, or perhaps their letters, in gold. Writers are given the gift of revision so that we may choose our words wisely. I plan to take full advantage of that gift. In the meantime, my speaking will probably remained flawed, but hopefully my character's speaking will not.