You are right that these phrases can sometimes be used interchangeably. But here are a couple of common differences: 1. “Be responsible for” often has the positive connotation of taking on a duty to help others, such as family or company or team. Example: “I am responsible for arranging all of our company’s trade shows.” “Answer for,” in contrast, often has a negative connotation of being blamed for something. Example: “He’ll have to answer for his failure to get the project done on time.” 2. “Be responsible for” often relates to ongoing or future duties. “Answer for”, in contrast, often relates to past conduct (as in the example above) or to “future past” conduct. Example: “If you fail to finish the project tomorrow, you’ll have to answer to the boss for that.”