Thou shalt not shine above thy fellow players
Most of us want our characters to be impressive, to have an impact on the people around them. And likewise, we want them to be strong, to be able to withstand the impact of personalities around them. If we succumb fully to this temptation, every character will be boasting and threatening... and everybody around them will be scornfully ignoring those boasts and threats.
Do you begin to see the problem? It shows up in many ways, but that is the most prominent.
Part of roleplaying as mature adults is learning to play nice with others. You need to be concerned with giving other characters (even those of the GM, occasionally) the chance to shine. Yes, you can take your opportunities, yes you should be most concerned with your own character. But you cannot be solely concerned with your own character, or you drain the fun from the game for everyone else.
Being considerate takes two forms. First, you should occasionally let other characters dictate the direction of play. If you're playing a rough and tumble warrior then maybe you can't respond with fearful snivelling to a physical threat... but you can respond with confusion and discomfort to a social or diplomatic situation, which allows the more sociable characters to shine. There are plenty of opportunities to be true to your character without dominating events, if you look for them.
Second, you need to make your actions in ways that other people can enjoy responding to them. It is all too easy to take an action that forces another character to either play second fiddle to yours (in order to maintain the drama) or to stall the game (in order to maintain their dignity). Now neither of those outcomes are really what we want, are they?