How do you decide how characters talk, walk, and move? What makes them unique in that respect?
Do they need to be unique? Yes!
What's the point of playing an average person who behaves average in every way? I'm already average, and I'm playing these games to be something different. For developing a character's outward appearance/demeanor, I consider the following acronym: SAGE STOP.
Okay, I don't really use that acronym, but those are the things that I consider. If I can place a character in each of those elements and think of how it affects his speech mannerisms, accent, walk, dress, or appearance in even one way, I've got eight factors that I can use to make my character an individual.
Let's walk through this using Professor David Edson, a teacher a Wildburry Academy (school for teenage spies), and one of my favorite NPCs I've ever made.
Social Status: Within his daily life, he's at the top of the food chain. He has superior intellect and skills, as well as a position of high power within the school. He has enormous authority, but also the responsibility of making sure each student comes out of the school totally self-reliant. This makes him come across as arrogant, stubborn, and cryptic, because he's constantly pushing the students to figure everything out for themselves. TANSTAAFL - There's No Such Thing As A Free Lunch.
Age: He's middle aged, so he's still got plenty of energy, and dresses like a professional. He's got a medium-tone voice which hasn't yet dropped into the baritone range as a result of old age. He has some strands of gray that's really starting to pepper up his black hair. He doesn't have wrinkles, but only because he never smiles. Speaking of free lunches, that high calorie food the school serves to make sure the students have the energy they need for daily physical training is starting to take it's toll. He's put on a little bit of weight - not enough to really slow him down, but enough that it shows in his face, his neck, and his waist.
Gender: He's a white man in a world dominated by white men. This gives him power, confidence, and privilege. He has a lot of cultural experience and has worked with many diverse people, but there's still a blind-spot there, and he realizes it. This might make him behave differently to students of color - maybe he pushes them harder in some circumstances, maybe he goes easier on them in others.
Education: He has a Ph.D. in Russian Literature from the University of Moscow during the Soviet era. No matter how hard he tries, whenever he speaks Russian he's going to speak with a Moscovite accent. It's going to color some of his word choices, even in English. His background in literature means that he is very well read, cultured. He has a broad vocabulary, and won't be afraid to use it.
Self-Perceived Wealth: He sees himself as a rich man. He has unfettered access to the resources of a powerful organization. This will make him ambitious, and sometime incautious. He's willing to sacrifice things, including lives, to achieve his goals.
Time: He grew up as a double-agent in the Cold War. Born British, but raised Russian. He sees himself as an embodiment of the best of both cultures - stoicism, unrelenting ambition, sharp intelligence, and morbid, dry wit.
Occupation: He's a consummate spy. But not only that, he's a teacher of spies who are still children. He has to be withdrawn enough to allow them to flourish in their own unique way, while still showing them that he is a force of nature and not to be toyed with. Children often fear what they don't know - so he doesn't let them know anything about him.
Place: For Edson, this one is pretty well covered by Time and Occupation. Place for him covers his birthplace of England, his formative years in Russia, all the operations he did across Europe for both the KGB and MI-6, and his current place in remote Romania. He dresses in layers, knowing that at any moment he may have to travel across the world. He knows that it's always more comfortable to take your coat off before sitting down in the airplane. He wears a mix of fabrics, and solid colors to be able to blend as unimportant in any climate and circumstance.
All-in-all, I visualize him as similar in appearance and speech to Jeremy Clarkson, host of Top Gear. But smarter. Sorry, Mr. Clarkson.