Brood silently is a phrase that describes someone deep in thought or worry, who is keeping the emotions to themselves. There are many different synonym options for this phrase, including introspective, contemplative, pensive, reflective, and deliberative. Each of these synonyms holds a slightly different connotation, so it is important to choose the one that most accurately portrays the situation. Being introspective would indicate a focus on oneself and one's own thoughts, while being reflective suggests a degree of analysis and thoughtfulness. Deliberative, on the other hand, implies a deeper level of consideration and careful decision-making. Ultimately, the choice of synonym should depend on the context and intention of the speaker.