Justin Kan, the founder of Twitch, a Y Combinator alum, and a long-time partner, says you have to get these three things right:

(1) Know what you are doing. You have to convince the interviewers that you are a great founding team, primarily by projecting confidence, and showing your expertise in the industry.

(2) Understand the small and the big idea. You should be able to convey the reasons why you are going to attack and dominate a niche area with your product, and then use that as a platform to successfully enter a much bigger arena. Expansion can happen in many ways, often by expanding geography, product category, or target customer. The important part is that you make a bold claim about how you will grow into something much bigger once you dominate your core niche.

(3) Convey founder synergy. More often than not, startups fail because of the founder conflicts. You have to show that you know how to work together, have clear areas of expertise/focus, and are able to resolve conflicts without sinking the company.