I'd also add a slight corollary: "Don't write Agent X because you want an agent of Agent X's cool and/or awesome species/sect/fandom/whatever, write them because you want to write Agent X". It's a principle that served me well from right back when I started - which hS took an active part in moulding.
Wobbles the Clown appeared because I wanted to do a genuine unironic clown, and up she popped; a happy-go-lucky sort whose happiness, luckiness, and (somehow) goness came about because of her previous life as an angstfic refugee who was intended to provide comic relief in the badfic itself but who was instead basically the chew toy of every electrical appliance in a given area. This desperate desire to make people happy (as well as the obligatory FicPsych sessions) made her into the clown we know and love today, but she was in need of a foil.
Then up pops something called the Continuity Council of Gallifrey, masterminded largely by hS and largely concerning the Time Lord agents of various oldbies, veryoldbies, and similarlygeriatricbies. I wanted to take part but, not having quite read the memo, bashed out a few syllables that sounded Gallifreyan and called the woman wearing them The Notary. When hS (via Morgan) pointed out that there were a lot of Time Lord agents who were Time Lords because it was cool, I wanted to refute that, so I began to think more about who the Notary was.
Now, in previous Whovian RPs on a Nintendo fansite that were frankly rubbish, I'd often used an OC Time Lord called the Gardener: righter of wrongs, snogger of aliens, part-time planet builder, and manager of a branch of Little Chef on the A504 just outside Milton Keynes. I liked playing as her, because she was fun to be around, and the original draft of the Notary was going to be much more like her. Then I thought some more about the Notary, and how I didn't want to play into hS's hands and give him a reason to boot me out of the RP... and that's why the Notary is the way she is.
It's detailed here and there in missions and RPs and ficlet responses, but the Fourth Notary was very much akin to my old Gardener OC in terms of temperament, drive, and weird hobbies involving greasy spoon management. However, I decided that that should very much be the past of the Notary, and then I remembered The Twin Dilemma, a Sixth Doctor episode wherein he throttles longtime companion Peri. It was under different circumstances, of course, but then I thought about what that would do to someone if they couldn't take it back, if that lonely old time traveller just let themselves rot in a decaying, unloved TARDIS. Suddenly the Notary's whole life story rolled out in front of me; this tired, defeated woman whose life has just been one indignity after another, until she finally got to be a hero... and then it was taken away, so she thinks. She was everything wrong with Time Lord society, everything that was rotten and corrupt about it... but, as Nirvana Crane once said (of someone else), "Somewhere under that pile of neuroses is someone nice trying to claw their way out".
From there, the dynamic with Wobbles practically wrote itself - Ronald McDonald paired with some ghastly little Napoleon from the local JobCentre has all the trappings of a normal agent pairing. So I decided to amplify those traits, really play up Wobbles' Pollyanna nature and the Notary's superiority complex, as a commentary on how limiting the standard hot-and-cold two person dynamic can be. Then Lola kept poking her head round the door, so I decided to bung her in as an agent, too - and then her story rolled out.
In fact, the only characters whose stories don't tie directly into that of the Oncoming Form are Doktor Trollenfisch und Gabrielle, who I wrote with the specific purpose of being soft and fluffy and having happy endings. Even then, they might not be separate from the rest of my agent stable for too long... though to say more would be spoilers. =]