@ Elizabeth: I run into that all the time! It happened with my Nano character, and I'm vastly annoyed with her. When I write characters, I usually try to picture how they look first. For some reason, this helps. That, and the theme of the story. That usually places their personality. I think about the circumstances, and I picture a character I'd like to see in it. Slowly they come together. Once I have their appearance, I can judge their personality. My characters are often insane or cocky, though sometimes I insert a shy, timid sort of figure. Or quiet standoffish. It depends.

So over all, I find judging appearance, setting, and theme to be the best way to bring out personalities. At least, that's how it works for me.

I have the same problem with characters!  I start out with them having distinct personalities but sometimes I forget things they're supposed to do and then they all are one sarcastic blob.

I also can't have a character that doesn't like to read.  I will have them not like to read just to have it as a thing and then I'm like "I- don't- know- what to do with you!  What are you doing?  Are you just sitting around?" At which point I give in and have them read.  It's not good.  Or maybe it is.  I don't know.

I write humorous things and I am horrible at keeping characters in character. I often deal with real people and I end up at a point where I want Character A to do ridiculous thing but Person A would never do that. I usually throw up my hands and say, "Let's do that ridiculous thing anyway!" and when people read whatever I've written, they complain that their character sounds nothing like them. 

Giving Main Characters Character: Really put yourself in their heads. Understand their motives and flesh them out a lot before you start writing. You'll most likely make changes to them throughout, but that's what your second and third drafts are for!

Minor Chracters: You know those characters who show up in one scene and then disappear forever? It's hard to keep them all different, and unless you want to write out enormous backstories for every farmer in every village, you'll probably end up with the same generic people over and over. If I want my characters to just pass some traveler on the road, I won't bother giving them any kind of character. However, if I want them to stop and chat, I'll give the minor character some kind of personality. Minor characters don't really have to be very realistic. I tend to give them one defining trait or feature that allows me to write the scene and then leave that character there without having the urge to go write about their backstories, too. Characters need advice from random old crone? I'll make her a swindler. Porter needs to open the door to let a character in? I'll make him old and with really bad puns. (I think I just like really bad puns.) Character needs to be turned into a dragon? I'll make a wizard trying to learn Spanish through the Rosetta Stone program.

General advice #1: Do your research before you write. If you're not sure about something, go Google it. 

General advice #2: Plan out your story entirely before you write. Don't just say, "Hmm, I wonder what will happen next?" I admit my Oedipus parody play is written in installments (every month or so I'll write a scene) and I have no idea what will happen next. This is bad, but at least I have a general framework to go off of: the Oedipus story. But then things like this happen, "Hmm, Ismene and Polynicus are randomly talking about plans to build a seafood catapult in Scene 9. What if they actually built it and used it? I'll write the war of Thebes after Oedipus leaves! Perfect! However, what sides should Antigone and Ismene take and how will the catapult be important? Ooh, I should stab Jem Louise too! And how about using Creon to summon Laius's ghost? Maybe even the Oracle will show up..." As you can see, this is very bad writing. Don't do it.