Speech without thought is like a body without a soul. A person who speaks without thinking utters empty words, and empty words don't have the power to cast a lasting impression on the listener's heart and brain. Therefore, if you want to influence others, it's a good idea to think before you speak.
The holy Rebbe Levi Yitzchak of Berditchev, of blessed and saintly memory said (Kedushat Levi, parshat Vayigash), "Words spoken from the heart penetrate another heart." In other words, when a person fervently believes and acts according to what he says, his words will have the power to influence others. Simply stated, we must practice what we preach for our words to be cogent.
The Yetzer Hara (Evil Inclination) tries to make us speak impulsively. Usually, the result of impulsiveness is remorse. When we allow our brains to filter the words that reach our lips, we don't make mistakes.
When someone asks you a question, you certainly have the right to pause for a moment, take a deep breath, contemplate your answer, and then say it. "I have to think about your question for a while," is always a legitimate answer. Any marksman knows that shooting from the hip is much less accurate than taking careful aim.
Shooting from the lip is even worse than shooting from the hip.