Character Differentiation

[Kasi Karra]

So I'm working on a story right now and there's currently 4 main characters (3 girls and 1 guy), plus 5 other supporting characters (only 3 of which are currently prominent).

When I write I like to stick myself in my character's shoes and try to think things out the way that they would, but I find that my characters are slowly starting to blend to be very similar in personality.

I was wondering if you guys had any tips or tricks that you guys use to keep your character's personalities different and distinct.  

[Priestess of Neptune]

If you do not already know the entirety of each of their respective histories and what has made them who they are, that is the place to begin. Even if the reader never learns everything that you know about your characters, they are not real people until they have a history.

If this has already been accomplished, then you might consider what each character's goals are. Without a general idea of what they want in life, they will seem flat, and without purpose.

If the previous two tasks have already been accomplished, then there are a few exercises which will help, although they may seem silly or strange.

1. "Be" a character for a day. Do not pretend to be one of your characters; actually be one. You shall quickly find out much about your character that you as the author might not have known.

2. Write a letter to a character. This is a standard Nanowrimo procedure, and may be very emotional.

3. Write a side-story for a character or two (not necessarily in the same book as you are working on).

(more later)  

[Hybrid Defect]

It is very important to give your characters a history whether or not the reader will ever know about it. I agree that writing a story with that character as the focus is a great idea.

One trick I find helps is to flesh the character out. Add defining details to them and think about how these details would affect their personalities and then their actions. With enough practice, they can take on a life of their own. Below is an example.
I have one character who is very religious but in a non conventional (comparable to protestant) view who is very jaded. He has been through some horrible things but tries to not let it haunt him. Another is a younger less experienced character who has a slightly naive view but a stronger sense of duty. He has also seen horrible things but is still shocked and offended when he still sees them.
When they both make it into town, the first goes to the bath house with a female character (who is a calm but savage) and a bottle of liquor while the younger character heads to a bar and grill and meets another character who wants to live a simple life despite their professions. As a result he feels bitter and often wouldn't mind a fight with the locals. The first man and the woman have personalities that feed off each other despite their differences and I am pleased to see how well they work together. Like in real life, the other two are left to make do with each other and develop a friendship of their own.
Groups of people, especially in pairs, can develop personalities just like individual people. The first pair become a sort of couple and take on an advanced personality where one keeps the other's negative traits in check while augmenting traits they desire. In the second pair, however, one person's negative attitude can spur the other do do things they normally wouldn't do like drinking in excess and looking for a fight with local townspeople.  

[Alexis Devoncroix]

I agree with Priestess of Neptune. "Be" the character. As cheesy as this sounds, sometimes I can't remove myself from one of my rp characters and I'll go the rest of the day in that mindset. But that does allow you to learn how that character might act in certain situations and with certain people.

Something that helps with me, because I have the same problem as you, is to talk to my character. Sort of like the letter idea, but I kind of have a conversation going on in my head, and it helps me get the hang of who my character is a lot better than sitting around trying to force him/her into being.