So how do we know how our characters think? Maybe you completed the character freewrite or interview exercises last week. Maybe you’ve filled out extensive character questionnaires. Maybe you only have a sketchy mental picture of a new character. No matter how well you know your character, you can help to make sure her thoughts—her voice, her feelings—come through in your writing in what she notices, how she talks/thinks about it and how she feels about it.
What they notice
My friend Annette posted the other day about “lenses.” She tells how on a visit to New York with her mother and sisters, they were each drawn to attractions that appealed to their personal interests—things that the rest of the family didn’t even notice.
Personal interests for your characters might arise from simply the need to “round them out” and make them more full, or they can influence the plot (she hates baseball? Fantastic—he’s a semi-pro shortstop.). When you’re just starting to design a character, even one simple interest can help to create deeper characterization.
Does your character have a passion for painting? Collect baseball cards and rare comics? Live for the dance? If not, why not? Everyone has something he loves&hobbies, interests, even their occupation. The architect might admire the layout of the museum while her dabbling-in-interior-decorating sister is more focused on the color scheme. Their wannabe-artist father, of course, is there for the art, while their hobby-egyptologist mother wants to hurry up and get to the mummies.
Our personal interests often filter what we see around us. The father in the above family might be the only one who really notices the paintings, but he barely glances at the dessicated bodies. These interests also influence our perceptions of those things that we do manage to notice.
Character vocabulary
A character’s personal interests, hobbies and especially profession not only filter what they notice, but the words they use to describe it—from the scenery to the events to the other people in the story.
I, for example, can’t tell a sloop from a schooner. But someone who spends every weekend on his sailboat is going to have a full vocabulary for not just every type of ship, but the masts, the rigging, the knots, the . . . other stuff.
Let’s say that character identifies himself, essentially, as a sailor, despite his day job in sales (*snicker*). When he meets a beautiful woman, is he going to think of her using the vocabulary of fashion? He might like the cut of her jib (that’s a sailing term trying to be a play on “fashion” and “cut,” not an innuendo), but unless she’s wearing a spinnaker (another sailing term—a sail. Very Little Mermaid.), I doubt he cares much about her dress.
Instead, he might use more . . . you know, “nautical” terms—the vocabulary of his passion. At this point, I’ve made it fairly obvious that I know nothing about sailing, but for lack of anything better, he might describe how she moves through the clumps of people like a cutter slicing through the waves. She could have eyes the color of the sea, hair the same shade as the burnished mahogany fittings of his cabin. (Okay, this dude is really starting to wax poetic for a guy, but maybe the sea does that to some people.)
The more parallels our character can draw to the things around him and his passions, the more likely he is to like those things.
Character attitudes
The character’s attitude toward the things and people around him is another important aspect of his character—and his voice. Perhaps most importantly, character attitudes are a strong characterization tool. When we see how someone feels about the world around him, we really get to know him. If he recoils at a church and quotes Karl Marx to himself (“Religion is the opiate of the masses.”), we know him more deeply than if the author just told us that “Jimmy hated religion.”
Again, his interests, hobbies and profession can influence this heavily. Our sailor friend might think a man whose only maritime experience was on a ferry to be a troglodyte. Put your 
character working in an urban environment. Freeway tunnels are the epitome of all that’s wrong with the city—they’re closed in, suffocating, dark, crowded, and most of all, nothing like the freedom of sailing, the open ocean, the wind in your face.
On the other hand, he loves taking his lunch on the observation deck of his building—when the wind is right, you get a breeze from the sea. He has an immediate affinity for people who strike him as sailors. And your Nautica bathroom decor? Well, you decide—he could either love the touch of sailing in your home, or he could think you’re a total poseur.
The slob might not even see the pile of clean (or are they dirty?) socks on the floor, simply walking past. But her neat-freak roommate is sure to notice—and she sees whether they’re clean, dirty, or a mix of the two—and then what does she think of her slovenly roommate? (Hello, Odd Couple!) If the neat-freak is a housekeeper or maybe a professional organizer, does she have a specific term for someone like her roomie?
What other ways can we incorporate and convey our characters’ voices?
Photo credits: 3D glasses—Harry Fodor; Sailboat—Horton Group; Anchor print—mckenna71
Start the scene in the POV character’s head as opposed to, say, using someone else’s (or possibly even the POV character’s) dialogue. While an interesting line of dialogue may make a good hook, it can also make it harder to figure out whose head we’re in, and how we should interpret that line. This is especially important in the very first scene 
Sometimes I get so into opening inside a character’s head that I make the mistake of opening scenes right inside their thoughts. For example, opening a scene like this:
But since our characters are individuals who are at least somewhat distinct from us authors, this can be a challenge. How can we ever hope to understand the thoughts and emotions of people who aren’t us? After all, we can’t experience the thoughts and emotions of our parents, spouses or friends—we can never truly understand exactly what they feel.
You could also try rewriting scenes from your story in first person. As you write, again, act, pretend. Close your eyes and visualize what happens in the scene, but not as someone else might see it—from the character’s vantage point. What do you notice? How does that make you feel? What do you think about?
Of course, that’s just third-person limited mode. What makes a point of view “deep” is how “close” we are to the viewpoint character’s thoughts. In a distant third-person mode, we may be privy to few of the character’s direct thoughts, and those are always related in italics. We may rely more on their actions and speech to characterize and understand them. Often, we’re acutely aware of what the viewpoint character is doing, as if we’re watching them with a tight focus, and every once in a while we get a voiceover of his or her thoughts (mmm, 
Sometimes, you can get so deep into POV that we don’t “hear” the “author’s” voice in narration, but the character’s. (And that can be awesome.) Everything we, the readers, get is as if we were seeing it through that character’s eyes (or brain, since we get a lot of his/her processing, too). We don’t just watch this character and his or her actions—we don’t see the character looking out the window. We see what s/he sees through the window. We seem to live the character’s experiences ourselves.