In my world preparation is everything, whether for trial, a transaction, oral argument or the first meeting with a new client. In your world there are equivalents where you can win or lose business based on your preparation or lack thereof.
I am heading into a trial in a few weeks and am in the process of preparing. While I think my client has a great chance of success at the trial based on the facts and evidence, I know that my level of preparation can greatly affect those chances and the ultimate outcome. My goal always is to be as prepared as possible because it not only gives me the best chance for success, but also the best chance to not miss something that can help win the day.
In this situation with the trial my preparation is compensated, but in many situations I am preparing by spending my uncompensated time. Why? Because the more prepared you are the better the possible outcome. If you think you are the only one your potential client, customer, etc. is speaking with, you are being naïve. And even if they are not talking to someone else, the confidence you instill in them because of your preparation will give you a better opportunity to reach the outcome you seek.
Plus, it can turn those people into your cheerleaders too, which never hurts.