How do I answer 'What will be your role in the US?' at my H1B visa interview?

Here is the most important thing to understand before you walk into that interview room. You do not need to know every detail of the specific project you will be working on. What you must know is the underlying need or requirement that your US office is trying to fulfil — and how your skills directly address that need.

For example, imagine your company has acquired new clients and there is development work that needs to be done for those clients. You are being brought in as part of the team handling that development. That is the need. Your expertise in that type of development is what makes you the right person to fulfil it. When you combine these two things in your answer — the company's specific requirement and your relevant expertise — you give the officer a clear, logical reason why you are going to the US. That is what makes an answer convincing.

How to prepare this answer before your interview

Before your interview, sit down and think through two things. First, what is the business reason your US office needs someone right now? Is it new clients, a new product, an expansion, a skills gap on the team? Second, what specific expertise do you bring that makes you the right person to meet that need? Once you have both of those answers clearly in your head, practise combining them into two or three sentences. Keep it concise and direct. A well-structured answer to this question will also naturally reduce the number of follow-up questions the officer feels they need to ask — which keeps the interview moving smoothly in your favour.