I'm not a natural roleplayer, and I'm introverted, so I have been in your place also. What I have done in the past is to select a TV or movie personality and model my character's personality on that personality. Most recently, I have adapted Survivor's Sue Hawk for my female dwarven barbarian/cleric and it has worked well for me. She is pushy, demanding, critical of everything, but very loyal to her husband, a dwarven barbarian PC. I have also used Survivor's Rudy Boerch personality for a dwarven defender.
Another resource that has helped me greatly in role playing characters who are not like me is the personality book
Please Understand Me II. It details sixteen personalities and they way they act and react to a variety of life situations. Not only have I been able to use this in real life to understand others but it has greatly enhanced my ability to role play characters in role-playing games. If you don't want to get the book, then using the astrology descriptions of each type of people born under each sign can also be a useful resource to model RPG personalities to.
After you have selected a personality, forge it into a character by answering the following twenty questions. They start off easy then get more involved as they go along.
- What is the color of the character's hair, eyes, and skin?
- What is the character's general appearace? Does the character stand erect or stooped? Dress anachronistically, stylishy, or inauspiciously? Look intimidating or casual? What might someone seeing the character for the first time think?
- Where was the character born?
- What is the character's age?
- What was the character's family life like?
- Has the character begun his own family?
- When or how was the character educated?
- Has the character ever done anything else for a living?
- What about the character's political and religious beliefs?
- What is the character's moral code?
- Does the character have any goals?
- What is the character's personality?
- What special qualities does the character posess? This doesn't necessarily refer to skills, but to what the character can do well. Does he get along well with other people? Is he skilled at taking care of business? Prioritizing tasks? Planning ahead?
- Are there certain things the character just cannot do? Get close to people? Perceive himself realistically?
- What does the character hate?
- What does the character love?
- What is the character's full name? Nicknames?
- How does the character perceive government? Those who are opposed to the government?
- How does the character interact with other members of the group? Are they protective of some members? Has the character developed a rivalry with another character? Are there any romantic interests the character would like to pursue?
- How did the character gain his abilites? What was his first reaction? What motivates him to act as a hero now?
After you have done this, see how you can integrate your character into the GMs game. Tie your character somehow to the major NPCs, organizations, region, and the plot of the game. Ask your GM if your character could be related to an NPC of the game, or if the featured organization has done something good or bad for you or your family. Try to think of ways on how and why your character is involved with the story or campaign. You may have to work backwards on this point by first determining what your GM is trying to do with his game, then create a charcter that would logically be in the center of all of it. Or just show your GM all of your character-building efforts and ask him to find ways to attach your character to his game so that you can be more personally involved in it.
If you are having a hard time tying your character to the game, either because the GM isn't willing to do so or he doesn't want to reveal what going to happen in the game, then tie your character to one or more of the player characters. That is what I did when I created my female dwarven barbarian/cleric. I selected a fellow player and decided that my character was married to his, after getting his OK for this. I asked him what his character was and build my character around his. Of course, there are other types of relationships. Siblings, cousins, parent-child, mentor-student, fellow apprentice, best friends who are also rivals, ex-friends, ex-couple, etc. This gives you a connection to another PC which can generate a lot of roleplaying opportunities. Plus, you have a player who has your back to a greater degree than the random stranger who joined your adventuring party.
These are the elements that have increased my partipation and enjoyment of role playing games, and have granted me some hard-earned ranks in playing roles that overcome my below-average Charisma and lack of roleplaying talent.