A character’s personality is created based on what they actually do. It’s all well and good to write “coward” on the sheet, but if the character tends to charge into battle, then the actual personality of that character is foolhardy or impetuous, not cowardly, and this arises out of actual play.
Through their actions, characters create a reputation. This is also part of their personality, and will affect social interactions. Charisma can help a bit (that is the modification to the reaction roll), but if you are Mao Zedong or Steve Jobs or Genghis Khan your actions speak louder than your charisma.
Basically, this is another way of rephrasing the novelist’s dictum of show, don’t tell or the aphorism that actions speak louder than words. You don’t need to speak in a funny voice or make up backstory motivations to play in character, you need to engage with the setting and show how your character is. If this means occasionally acting against the incentive system of the game, it is exactly that counteraction that gives the role playing weight (just like altruism is not actually altruism if you are paid for it).