1. Character Is Action.
I cannot stress this point enough. Character. Is. Action. Characters do things throughout a story, and by doing things I mean they don't sit around waiting for the story to happen to them. Characters thrive on achievements and goals [we'll get to that shortly] and should be on the whole proactive about meeting those goals by interacting with their environment, other characters, and overcoming obstacles. Character is revealed and further built upon through action. It's a delicious, nutritious cycle.
How do you create action? Show, don't tell. Don't get lost in exposition of what your character is all about. This is boring and, to some readers, offensive. Most writers and readers are capable of scoping a character based on what the character does, based on what they show. You can tell us all day long that your character is kind-hearted and friendly, but until it's proven through action then the telling means nothing.
"Bobby is kind-hearted and friendly."

Oh really? How do we know this? What actions has Bobby provided to show us that he is kind?
Showing is providing evidence of the claim.
If you get to post #3 and your writing partner can tell you what they know of your character based on what they've seen, you're doing swell. "Bobby is kind-hearted and friendly because he's nice to his brother and, gasp, politely conversed with Jenny after she accidentally bumped into him!" Through actions, readers are made audience to Bobby's qualities of kindness and amicability, not told. If you have a mean character, make him say or do something mean to another. If you have a cranky character, make them complain about something. Don't tell us he's grumpy, have him do something to show it. We'll get the picture.
Remember: if you have to tell people what your character is about, you aren't characterizing properly.
A Grandmaster doesn't call himself Grandmaster, he is called Grandmaster by others.
2. Character Goals Drive Story
Characters without goals are boring and painstaking to write with. If I get to page three and I'm thinking to myself 'where the hell is this thread going? Why are we here? What are we doing?' then one, or both, of our characters are lacking something essential.
Goals can be big: Bobby wants to become a Jedi Master. Goals can be small: Jenny wants to find this book. Whatever the case, when you begin a thread do so with a goal in mind, or even a few. Goals provide stakes for your story, as most goals, no matter how big or small, will often [and certainly should] be met with conflict and obstacles. This brings us to a writer's most important equation:
Character + Goal + Conflict = Story
Once you've decided upon a goal, don't tell us what it is - show us [curse you tip #1!]. If Jenny wants to find that book, she better be on her way to a library, museum or archive, and when she gets there you can show us how important this book is to her by providing some sort of conflict to develop the story. Ask a friend or even, flinch, allow a stranger to intercept.
Jenny arrived at the archive only to find that Bobby has taken the only copy of the book - and now Bobby was two star systems away. Without pause, Jenny immediately ordered her ship fueled and ready, she had to find him and that book.
Wow, clearly this book means a lot to Jenny and now there's a nice juicy story flowing. As the reader we have also discovered that Jenny is quite a determined and perhaps even stubborn character, and we weren't even told. Congrats Jenny, we're super interested to find out what happens next!
Remember: Goals are the gas that drives the engine of the story. If your story is stagnant and your readers/partners are bored, reconsider and reestablish the purpose and goals of the thread.
3. Details About a Character Should Have Story Significance
Details about character appearance should be treated like the weather. Tell us it's sunny or raining and get on with it. Not only does this make the thread easier to follow, but also easier to respond to. There's nothing worse than being confused by your partner's post because it has nothing to do with the current story. Unless the lacy, frilly dress has some sort of incredible significance to the story and the plot hinges on it's beautiful opalescent buttons, I don't want to hear about it and how it elegantly sweeps across the floor for four paragraphs.
[Learning how to artfully describe something will be touched upon in another blog entry.]
The same can be said for things concerning the character. If you spend three posts describing Jenny in a gymnastics routine, it better have something to do with how she defeats Bobby to claim that fething book from him or I'm going to be pissed. Don't waste your reader's time with superfluous dribble and for the love of the Force, don't tell us the same thing over and over. If you absolutely feel the undeniable need to describe Jenny's outfit in your opening post of a Gala Event, do not do it again in posts 3, 7, 12 ...
Post a linked picture for God's sake and move on. Reader's are capable of maintaining your character's image how it suites them. Taken from an old RP friend of mine:
The character is imagined the way the reader wants to see them, and regardless of how hard you try, it cannot be put into a box of meticulous description. And then if you have a reader like me, aching with bitterness and frothing with loathing for all things written, I’m thinking “Does his hair mean something? There’s no possible way his hair can mean something! But they’ve spent a paragraph on it!” ~ Laura Jennings
Remember: partnered writing makes it difficult enough to keep the cohesion train on its tracks without adding meaningless things. Write to what is relevant and important for your current story and stick to it.

4. Define and Play to Your Character's Strengths and Weaknesses
Strengths are often one of the easiest things for us to define. Typically most characters are crafted around their strengths - which makes sense. We spend our lives honing skills and traits that provide us a positive step in the direction of our survival and our happiness. Strengths come in the form of physical, emotional, and the intangible elements of ability. Your character's stories are often built upon their strengths, but must be equally balanced with weaknesses - otherwise the story will lack conflict [refer to Tip #2].
In terms of character, especially while in the process of character creation, weaknesses are often forgotten and overlooked. I cannot tell you how many character bios I have perused only to find that those weaknesses written however long ago never seem to crop up in the story. Often weaknesses are physical limitations or aversions, such as an allergy or inability to perform or manage certain tasks. Weaknesses are typically part of the blueprints of a character, within the foundations of its creation. A good, solid weakness cannot be overcome by the character itself except for the instance of deus ex machina [which is a literary no-no, kiddies] or, preferably, through some very extensive character development, evolution, and gasp! story. Weaknesses present themselves as conflicts within story. Weaknesses are important. Not only do they make your character more relate-able, but more interesting and likeable too.
Character Flaws are another form of weakness. Flaws are often manifestations of a character's means to cope with their weaknesses, and can even be side effects of their strengths. A flaw is something that can be overcome by the character itself through story and development.
For example: One of Jenny's strengths is that she is very analytical, but in this strength therein lies her flaws: she's a strong thinker but omits emotions from the process which often leaves her disconnected from others; she's very thorough but this means she can go too far and may agonize over perfecting things; she's disciplined but this caries over to being too demanding of herself and others.
All of these things can be overcome by the character. Perhaps not on her own and not right away, but a good story with supporting and even antagonizing characters can help Jenny to see her flaws and overcome them, or not, if she happens to be the sort of character that doesn't care...which is a good way to show us that Jenny is cold and apathetic without having to tell us. See what I did there?
So how do we bring all these things together harmoniously through story? Once you've balanced out your character with strengths and weaknesses, be sure to provide them in your story. If Jenny is a talented swordslady but she's deathly afraid of heights, I better not read about her trouncing Bobby atop a Coruscanti skyscraper without so much as a blink at her surroundings. Use that fear to provide more conflict. I'm not saying she cannot manage to overcome it enough to get that book from Bobby, but feth, wouldn't it be a better story to see brave little Jenny piss her pants after realizing how high up she is and have to deal with some soggy drawers in an epic duel? You betcha, and rest assured no one will forget about that particular scene.
Remember: no one is perfect. Your character is not a Mary Sue or a Larry Sue. Society thrives on the flaws and weaknesses of others and the drama they beget. Readers love a character that fails. Readers love a character that struggles. Because when your character fails and struggles, it makes their achievements that much greater in the end.
5. Know Your Character's Limitations Within Their Realm of Existence
This goes for everyone from the humblest of Padawans to the grandest of Mandalores. It's not a difficult concept to understand, but almost everyone is guilty [including myself] of handwavium when it comes to culture, traditions, laws, and even just rules of physics. Much of this can be linked directly to your character's strengths and weaknesses, and a lot of those things may also be described and defined by their race, culture, local laws, guild rules, etc.
A Padawan Learner shouldn't be galavanting across the galaxy, spewing Force Lightning at his foes and burning fuel in a big shiny ship. Padawans, canonically, don't make money. They depend upon the funds and generosity of their Order and the guidance of their Masters. Padawans do not contain the wherewithall to summon Force Lightning nor the practice and skill to maintain it to smite their enemies. Allow your character to live up your Padawan years. Let them be eager to learn, incapable of handling every task presented, honest with their knowledge, and weak in their abilities. This is your time to develop your skeletal frame into the character they're going to be some day. [Obviously there are exceptions to this train of thought, but I'm speaking in general.]
If a war is brewing between two factions and Jenny needs to cross their territory to get home, it's likely she'll run into a few roadblocks along the way. Just because Jenny is a ranking official in minor faction X doesn't mean she has free reign through space. She very likely could be captured as a POW, or detained for questioning while crossing borders and possibly groan lose her book in the process! It is not within Jenny's means to simply dismiss the limitations plot has set for her. Now Jenny needs to call upon a favor from Bernard, and he's not gonna do it for free.
When contesting limitations, be prepared for the consequences of it. A brave Sith may test his mettle to prove himself against his Master, or a lowly soldier might disobey an order from his superior, and both of them better be ready for the backlash and the conflict that arises. Hello, conflict, come here often?
Cultural limitations might dictate your character's behavior in certain situations. In Bobby's case, a man from a background of forgiveness and generosity, might defeat Jenny in that high-rise cityscape duel, but in the end he may still give in to her wants even if he had need for that book himself. This selfless act is perfectly in accordance with his upbringing, but his Order may frown upon it when he returns home without the book that was tantamount to completing his Master Trials. Now he'll face the repercussions for letting an obvious psychopath go and will need to find another way to continue forward on his path to becoming a Jedi Master [obstacles! conflict! ;alfjaljfeif;anf!].
Physical limitations are some of the easiest to write to when one fully understands and utilizes their character's strengths and weaknesses. But there are other things you might not think about - smaller details that pertain to your character's physical self as per their race or history. A bad knee? How about a character who might be missing some toes? Both of which would effect the character's ability to balance and move. Does your character have opposable thumbs? How about an opposable tail? Does she only eat a certain type of food that's hard to find anywhere but her home planet? Hair length, too, is something often overlooked. If you've never had long hair then you don't realize what a nuisance it can be during daily tasks. It can hinder eyesight, it can get caught in and on and around and under all sorts of things, it can be terribly heavy when wet, it can cause headaches if styled certain ways ... the list goes on.
Remember: Every character is bound to laws and limitations within their sphere of actions and self. Use them to your advantage for weaving character development - they'll only add to the richness and intrigue of your character's story.
That's all for today. Now get out there and write.