Exercises for the Mind
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       Here, I have some excercises that may prove useful for character development :)  Some of them might seem a bit odd, but one needs to be a bit odd to be a writer ;) These are in no particular order, so feel free to take these as you will :)
Activity 1
  A helpful thing to do, to help with the realism of your characters, is to create a height chart.  In our heads, these people are larger than life and putting up a physical reminder of just how big they really are is very helpful.  I've found myself quite surprised at how much smaller most of my characters are to stand next to than I expected.  This also makes it easier to place them in crowded environments realistically.
Chart
Activity 2
  When listening to the radio, it's a good idea (though many do it without trying) pay attention to the lyrics and find songs that fit your character.  I've been able to make a whole tape, front and back, of songs that relate to my anti-hero character alone, in various stages of his life.  Also, as suggested in Details, it helps to notice what songs your characters like too.  Let them speak to you :)
Music
Activity 3
  Now this may sound odd, but I've found it to be helpful nonetheless as have some others of various religious backrounds.  The tarot is not something people should run their lives according to, but learning it and using it as a means of creative inspiration is both harmless and a handy writer's tool.  Especially when a character has you stumped or you've run into a case of writer's block.  Visit A Full Deck where you can search for the right deck for you.  Whichever cards inspire you is best, and the cards rarely fail to get you thinking and the creative juices flowing again.
Tarot
Activity 3
  When you're watching movies, TV, or reading books pay attention to the characters you like and dislike.  Be aware of how you feel about them and why you feel that way.  Is it because you can relate to them?  Because they are funny?  Do you dislike them because of who they are, or because they seem flat and unreal?  Observe and contemplate what works and what doesn't, as well as the chemistry between characters.  What personalities mesh well?  What personalities clash?
Observe
Activity 4
Use public transportation :)  You're bound to observe and/or meet lots of interesting characters, and come across lots of great inspiration.  Take a long walk, sit in a park or some public place and watch the world go by.  Imagine what people may be thinking, what their lives are like.  And don't just observe, interact sometimes too (where appropriate, naturally) meet people, experience life.  A writer ought to be the world's sponge.  It's a great way to learn.
Experience Life
Activity 5
Astrology, like the tarot, is another useful tool for inspiration.  It can help round out a character's personality.  And I'm not talking about horoscopes at all, but basic personality traits based upon the Rising, Sun and Moon signs.  (If you're zealous (crazy) enough to do a whole chart, more power to ya.) You can use this one of two ways ... Look it up and apply which signs fit a character you already know, OR if you're making up a new character you don't know yet just give them signs and use them as a guideline to help you further develop them.
Astrology
Activity 6
Animal symbolism is another good place to find inspiration for delving deeper into a character.  Think in terms of totems.  What animal's symbolism, mannerisms, ect fit your character?  There's many good resources for this method of study, including the book Animal Speak by Ted Andrews, and many different sources online.  on my links page you may find some helpful sites.  For example, my most beloved character's totem is the skunk, and if you really research the shamanistic symbolism and mannerisms of the skunk ... you'll know a lot more about this character than I'm sure he'd like you to ;)  My own personal totem is rather easy to guess, so I'm not going to insult you by pointing it out.  But I've found that this method of character exploration is very helpful, and especially fascinating for animal lovers like myself ;)
Totems
Activity 7
  When you think you've really gotten to know your characters, an enlightening challenge is to explore not just how they react to different situations, but how they react under completely different backrounds.  I've found this to be one of the most fascinating activity for both character development and writer's block ... Exploring past lives and "what ifs".  See how your orphaned streetwise nightblade may have been different if, say, raised in a wealthy loving family and as a different gender. 

   It's also an easier way to try out different genres ... by taking a beloved character and dropping them in a totally different body and environment.  For example, I tend to lean toward fantasy/sci-fi.  But I experimented, taking my favorite character (an anti-hero very close to my heart) and dropped him into a gritty western ... a genre I'm neither fond of or familiar with ... as a womyn.  That life didn't go so well, but I suppose being a pregnant outlaw never is, but interestingly effected his
next life, in which he was a modern Detective (male again, and in another genre I don't know well)  and a clean-freak to boot.  These are just a few examples, and I've learned a lot about my beloved characters from these karmic experiments.  Like how complicated ethical and life choices are to some... and how others don't seem to change much no matter what you do to them.

   Again, as with the tarot, this is a useful activity for those of any religious backround, primarily because it is being used for fiction.  You don't have to believe in reincarnation and Karma to toy with these ideas. 
Explore