Voice: It Needs to Fit

Eek!! I'm sorry for disappearing for so long. Around my house, everything has been crazy!!!!
Hey! Let's use this as an example. :)

Instead of what I wrote above, what if I had written:
Oh, my. I am so very sorry for have disappeared for so long. But where I live, we have been very busy.

Or:
Ah!!! Sorry I went poof! It's crazy 'round here.

I could go on, but you get the point. :) Voice.

Should you make your well-educated, brainy character say, "Ain't this a purty day?" NO! (Unless the circumstances call for it, of course.) "Is this not a beautiful day?" would probably be more appropriate for them.

You're probably all rolling your eyes, thinking that this is unnecessary. But it isn't! Dialogue is one of the most important parts and if your characters don't talk how they should, you have a problem.
This does not mean that you should make your characters talk as fancy and as good-of-grammar-ish as you can make or as bad-of-grammar, uses a lot of slang, etc. There's some type of happy medium. However, you might have circumstances where this will be necessary.

In addition to how your characters talk, you have to think about how much they talk.
I'm an introvert. I don't especially like small talk or talking or a lot of conversation. So it can be harder for me to write for my extremely extroverted characters. Extroverts might have a harder time writing introverted characters. But it has to be done.
Some of your characters will talk a lot, some of your characters will be very quiet, and there will be all types in between. So the key is practice, practice, practice!