The #1 rule of answering this question is doing your research on what you should be paid by using sites like Payscale and Glassdoor. You’ll likely come up with a range, and we recommend stating the highest number in that range that applies, based on your experience, education, and skills. Then, make sure the hiring manager knows that you're flexible. You're communicating that you know your skills are valuable, but that you want the job and are willing to negotiate.
Any candidate can read and regurgitate the company’s “About” page. So, when interviewers ask this, they aren't necessarily trying to gauge whether you understand the mission—they want to know whether you care about it. Start with one line that shows you understand the company's goals, using a couple key words and phrases from the website, but then go on to make it personal. Say, “I’m personally drawn to this mission because…” or “I really believe in this approach because…” and share a personal example or two.