Toremeggen marches with a heavy gait, favoring his left leg from where he risked his hide to rescue a unicorn from a massive python. LeCyndi teases playfully but is always on the lookout for 'the one that got away.' Tyrius is quick with his wit but even quicker to flee the scene – even when he's not the one to blame. What's the difference between a few statistics on a character sheet and a creature with vigor and definition? The only one is in how the player sees and explains them to the others.
Suppose you've got a sword-master. Their attributes are all geared toward combat and little else, perhaps. Should you just keep quiet when the group gets into town? Not at all. Role playing games can have a lot of combat, which many of my friends enjoy more than anything else, but at their core is "role playing." Being a walking sword just waiting for the next thing to stab is truly robbing yourself of a grand experience. At the very least this character could behave impatiently if they haven't gotten their way, right? On the one hand there's a mute with no opinions, knowledge or perhaps even a frontal lobe. On the other hand there's a restless warrior who paces and raps against the table when the other players take too long in town, urging them to hurry it up.
But how do you role-play a character with no diplomatic skills or barely any intelligence? Those are the most fun, actually. Essentially that question is better rephrased as "how can you role-play a character when talking isn't their strength?" Pick a flaw, any flaw, that you've observed others do. Maybe even one you used to do. One of my recent characters had an abysmally low intelligence and he would use words he didn't know the meaning to (wrongly, most often) and make up words on the spot (really bad ones). Acting out strengths such as combat can be fun but so can acting out weaknesses. This same character almost got to enjoy his element of fighting by acting out his most blatant weakness – until the group's holy-roller realized that he wasn't mispronouncing his patron deities name on purpose, anyway.
There's two types of characters most commonly played. Characters who are very alike to the players portraying them and characters who run opposite. While role-playing myself is fun (and simple) it is also a lot of fun to pick other character concepts and run with them. Have you found an intriguing character in a book? Let them inspire how your character responds to others or how they hold themselves. If you're lucky like me then you have an interest in behavioral sciences and psychology. I visit a few websites here and there to learn about different things people do and very often one or two of these ideas will spawn a character concept all it's own. Perhaps while I'm reading about looking glass personalities I want to make an illusionist mage who never fits in anywhere. Another example is histrionic personality disorder – that's a rogue with entitlement issues and a princess complex. Underlying both of these is finding inspiration wherever you can. Billions of personalities exist in the world and any role playing game universe should be no different.
A favorite tactic of mine is to run with archetypes. A fighter who stands for honor above all things or a mage who values empathy before all else... These don't necessarily reflect every choice they'll ever make. If you have a good idea of where the character is coming from in contrast (or alliance) with where you as a person would be coming from then their choices become easier to spot in the grand scheme of the story. Playing to the other side of the coin, megalomaniacal wizards run in all shapes and sizes as well as shifty-eyed pickpockets. Remember that whatever traits you pick for your character don't have to define all that they are. Introducing dysfunction into your adventuring party should always add to everyone's fun instead of detracting from it. Traits are just a nice starting place.
I've met a new gamer or two who believed that they had to create and iron out their concept before the game. Because of course people never change, or do we? This bears repeating – as human beings we are not static creatures. Decisions and events pave our lives and shape our personalities every single day. Some things are a lot more resistant to change and this can be both good and bad. I believe I'm very different from who I was two years ago, five years ago and ten years ago. Let the character's personality emerge throughout the course of the game and most especially don't be afraid to make mistakes and be silly. So what if your cynical warrior just decided to trust someone they just met? Maybe they had a momentary change of heart, or this person was just special, or something tragic might happen as a result and force them more strongly into their ways of mistrusting everyone.
You should remember when breathing life into your character that it's not a constraint but an outlet. When engaged in role playing games we get to explore not only uncharted dungeons, massive castles and the political intrigue all around the characters. People are everywhere (usually). Look at the friends in your group, you'll be adventuring with them for a good long time hopefully. However you paint your character to act is how their characters will know them. Make it a colorful tapestry!
If you can strongly see your character and have real definition invested in it then the rest of the group will see that over time. Instead of telling them "Jimmy is a man of few words," show them by selecting Jimmy's replies concisely and speaking only when Jimmy might deem it appropriate. Is Talia really a vicious monster seeking revenge on the person the other adventurers are questing to rescue? Then it'd probably be much more of an interesting twist if Talia was the most polite person in the party to compensate, wouldn't it? Character concepts, roles and dialogue are truly the heart and soul of the role playing game.