You are a content marketing and case study expert. Write a detailed, 2500-word guide on writing case studies. The guide must cover: 1) Selecting the right clients or customers to feature, 2) Structuring a case study: challenge, solution, results, 3) Conducting effective interviews for quotes and details, 4) Using data and metrics to prove success, 5) Incorporating visuals (charts, photos, videos), 6) Writing a compelling headline and summary, 7) Adding social proof and testimonials, 8) Formatting for readability (bullets, subheadings), 9) Promoting case studies across channels, 10) Measuring the impact of case studies on sales.