Dialogue is one of several tools that an author uses to establish characters and make them distinct from one another. If you're writing fanfiction (or parody, or pastiche), getting the voices of your borrowed characters to match the author's original renditions can be as important as making their physical descriptions accurate. Still, it's likely that all of us have had the experience of writing a scene in which our favorite characters simply don't sound like themselves. How can this situation be remedied?
The only real way to get a feel for how the voices of characters differ is to take a look at samples from the original work. For example, in the classic young adult novel Little Women, the core cast consists of four sisters: ladylike eldest sister Meg, brainy tomboy Jo, gentle music-lover Beth, and lovely youngest sister Amy, who tries to make herself seem older by using (and misusing) uncommon words. Here's a sample exchange from the first chapter. Can you tell who is speaking?
"I detest rude, unladylike girls!"
"I hate affected, niminy-piminy chits!"
Meg is very ladylike and tries to be mature: it's very unlikely that she'd make either of these angry statements. Likewise, Beth is known for rarely disagreeing with her sisters, let alone arguing. Alcott has made it quite clear that the first speaker is Amy, and that she is being answered by Jo. Notice that Jo uses straightforward language: hate rather than Amy's more affected detest, because she doesn't mince words and has no reason to try to make herself seem superior by using advanced vocabulary. Even in a dialogue this brief, it's possible to establish character firmly.
The manga Saiyuki also has a main cast of four: the taciturn, grumpy priest Genjyo Sanzo; his enthusiastic young companion Son Goku, also known as the Monkey King; the brash, flirtatious, and soft-hearted half-demon Sha Gojyo; and mannerly and monocled healer Cho Hakkai. The differences in their voices may not be as easy to pinpoint in English as they are in mangaka Kazuya Minekura's original Japanese, in which readers can notice, for example, that Hakkai is the only one of the four who uses the polite form of the first person personal pronoun. However, the translators (sisters Alethea and Athena Nibley, who also translated Fruits Basket) and the adapter (Lianne Sentar) still make solid attempts to give distinctive voices to all four. If you take time to read some of their dialogue analytically, you'll notice some of the techniques that they have used.
For example, to cover Hakkai's educated Japanese, the English translation gives him the most advanced vocabulary in the group. Words like atonement, anxiety, and epiphany fall easily from his lips. He's likely to use the neutral noun one in an example rather than the familiar you: "Reading the flow of the game can only take one so far," he remarks, of his skill at cards. He's also the most likely to add a tag question after his statements: "We haven't seen any youkai along this road, have we?" He is precise (if voluble) in his speech, saying yes rather than yeah, no rather than nah. When he addresses strangers, he's polite, using titles such as sir, miss, or ma'am. And in contrast to at least two of his companions, he almost never swears. When he says "to hell with it" in one scene, it's meant to be startling.
In contrast, Hakkai's very close friend Gojyo, who has led what he himself describes as "a punk's life," speaks very casually. The English translation has him use uneducated grammatical forms such as ain't and U.S. slang terms such as dude and chick (for man and woman). In contrast to Hakkai's precision, he slurs, truncates, and elides words, leading to forms such as c'mon and wanna. He often calls his acquaintances by mocking or teasing nicknames, and he addresses strangers with familiar, casual terminology, such as buddy, sweetheart, or auntie. And he swears constantly, not just when he's angry. Gojyo swears for emphasis, as punctuation, and for the hell of it.
It's easy to pick up similar patterns in the speech of the other two. Sanzo tends to be terse, he almost always sounds angry, and he swears frequently, but he can also give long, serious speeches that prove that his education has been every bit as deep as Hakkai's, if not as broad. Goku's speech can be even more casual and imprecise than Gojyo's, but he doesn't swear: Goku is meant to be quite innocent in a lot of ways.
In this excerpt, our heroes have arrived at an inn and have just been promised dinner in their room. They are disguised in long, hooded cloaks and dark glasses in hopes of eluding their enemies and are using assumed names. I have had to put in the framing bits, because this is manga:
"Yaay!" crowed Goku, happily, as they headed up to their room. "It's been forever since we ate for real!"
"Curb your enthusiasm, shit-for-brains," said Sanzo. They continued on in silence until the door was shut between them and the rest of the inn. Gojyo dropped into a chair and fumbled for his cigarettes and lighter. "Damn. How much longer do we have to wear this crap?"
"Quit your whining," said Sanzo, pulling out his own smokes. "We wear it until people stop freaking out when they see us."
"We need to preserve our cover for now," added Hakkai. "Please be patient."
"Feh. Fine. But it's 'Dear Brother Taro' who's screwed the most if he's seen," said Gojyo, looking meaningfully at Sanzo.
Sanzo lowered his lighter and glared: "You're inviting nothing but death."
"Your willingness to cooperate is appreciate ... 'Big Brother Jiro,'" said Hakkai, sweetly.
"Look," said Gojyo. "Jiro here doesn't doesn't know if he can -"
"Stop nibbling on anything with girl parts?" suggested Sanzo nastily.
"Hey! I have refined and discriminating tastes, jerkwad."
(For those who know this scene: yes, I have left out the reactions of little Seika, who is eavesdropping in the hall outside their rooms and drawing all the wrong conclusions.)
In the manga, Minekura often wraps a story arc with a comic scene that features a long shot of the four protagonists driving westward in their Jeep, with word balloons around the perimeter for each character. Their voices are distinct enough that it's possible to tell who is saying what, even though one can't see which balloon is coming from which tiny figure. The same can easily be done in fanfiction:
"We've been driving forever! How much longer? I'm starvin'!"
"You lying little monkey! Hell, you're not starving - you still take up just as much room!"
"If I hear another sound, our load will be lighter by two corpses' worth."
"Oh my, this is serious, isn't it? I advise silence until we reach our destination!"
The differences in characters' voices can also be far more subtle than these examples. For the most recent Yuletide challenge, I was writing a story based on Rosemary Sutcliff's 1954 YA historical novel The Eagle of the Ninth. My Beta reader noted that I was using nearly identical voices -- slightly formal British English, as I thought I'd observed in the novel -- for both former Roman centurion Marcus and his British slave Esca. She suggested a few very small changes to some of Esca's dialogue on the basis of her close reading of his scenes in the original. For example, near the beginning of my story, when they are about to leave on a long journey, Marcus announces that they are about to depart, and Esca says that he's going to get Marcus' tame wolf:
"I will go get Cub, then," said Esca, and slipped off to the stables.
My Beta suggested that I change this as follows:
"I should be fetching Cub, then," said Esca, and slipped off to the stables.
This slight change in phrasing, which suggests just a hint of a Scots or Irish accent, makes Esca's voice clearly his own and echoes Sutcliff's treatment of his speech in the original work. Now that I was aware of the difference, I was able to go back and rework bits of Esca's dialogue throughout so that his fans would be more likely to feel that he was indeed the same character they had loved in the novel.
It's true that it's easier to analyze dialogue in written materials. If you are working with a video (or audio) medium, it can be worth seeking out scripts, screenplays, and transcripts for examples of your primary characters' dialogue. As one of my fellow Grammarians pointed out, even the quotations on sources like the Internet Movie Database (IMDb) can be helpful in pinpointing what's distinctive about a character's voice. If all else fails, review video clips and make your own transcripts for lines that sound very "them," so you can take a good look at what's really going on.
So let your source's author -- or scriptwriter -- teach you how these characters talk. With a little care, your readers will be able to respond with a pleased sigh of recognition, rather than a puzzled frown!