I have been in this scenario quite a few times. Depending on your skill level and personality you can be either the weak link or the driving force of the team.
Talk to the product owner. Understand the requirements. Clarify the stories. Ensure that everyone is on the same page with the exact same expectations. Your primary task should be as an advocate between the Development team and the users. Ensuring that neither Dev nor Users is chasing any golden gooses and that everyone knows their role.
Find defects before they exist
Talk through every story. Allow the product owner and the Developer to discuss how they envision the end product. Develop a list of potential issues or tricky areas and ensure that they are covered prior to development. Make it clear that the developer not only can come to you for any questions but that you will happily assist them in any way through the process in order to create the best possible solution.
Study study study
Know your product, inside and out. If you have dev experience, do code reviews regularly. If not, learn how each piece functions (do this with dev experience as well). Learn the people on the team. Learn how to manage the Users/Owners expectations. Learn your Developer's strengths and weaknesses. Work closely with everyone and earn their respect.
Once you have established yourself within the team as someone that can be relied on people will naturally look to you for the solutions to their problems. This opens up the ability to ensure that issues are corrected properly prior to their creation which is a huge win for everyone.
Expect a lot of work early on depending on the status of the project. Your main goal currently should be to take the knowledge from everyone in the team and consolidate it while learning the patterns and abilities of your colleagues. This will usually mean long days and potentially nights as well but if you achieve it, it will become one of the best jobs you've ever had and is extremely rewarding when you start getting consistent and constant successes.