letters

You know what are awesome?  Letters.  Not the alphabet, not emails, letters.  They can say a lot about a person, I think.  For example, if they're bothering to write down the stuff at all, that makes them awesome.  And if there's a lot of cross-outs, they probably doubt themselves or something.  Or they just are writers and need to find the EXACT WAY TO SAY SOMETHING.
A letter is also a good literary device.  Again, it can tell a lot about a character.  What do they say in this letter?  How much subtext can be implied from this?  How much subtext can come from the words, and how much subtext can come from the action?  What do you think the motivation was for writing someone the letter in the first place?
Most importantly, is it a letter of love, or something else?  I mean, a lot of time friends write to each other.  I got a letter from my youth group leader yesterday with a lot of advice for college and life and stuff.  I haven't become pen pals with my boyfriend yet, but it could happen if he'd give me his snail mail address (*looks significantly at him*).  You could have your villain write a nasty and provocative letter to your hero(ine) if the villain knows that it will goad the person into saying or doing something that's utterly stupid, but may or may not serve the villain's plans very, very nicely.  If it doesn't serve the villain's plan nicely, then it at least should probably play to the weakness of the recipient if it's from the villain.  The truth is, if you write fantasy, like I do, most of the time the villain is there to stir shit up (sorry for my language, mom) and do little else.
But there are also character things that need to go with letter-writing.  You can't have people be pen pals if it's a dystopian society where pens, paper, and pencils no longer exist.  They could be e-mail pals, sure, but it doesn't serve quite the same function.
Additionally, if the sender is not one for communication, they should only be recording their thoughts and mailing them off in one of two situations:
1. a massive confession that can considerably change the course of the plot/ the recipient's feelings/ etc
2. NO OTHER TIME.

Naturally, 1 is a broad spectrum, but if it's out of character for someone to take the effort to write something (or type something) and send it to someone, don't have them do it.  If it's part of the character arc, go ahead.  But if they don't like to write unless they're forced, they probably shouldn't be asking around for a pen pal.