While this is an interesting question, it depends on the developer, I would like to give you some guidelines! You may be familiar with what some developers would need, such as a web or house developer, and the intent is the same when talking to a product developer. The goal with any communication with a developer is to communicate your needs as clearly and concisely as possible. What you need varies depending on the project. Because of this, I’ll break down what you’ll want to bring for a couple different project types. Basically, what best explains what you need, when you need it, and how you’d like it done is what you bring to your developer.
For most firms there is an application process of some kind, a questionnaire, form, interview, etc. that asks the questions to best get that information from you to the developer. In addition to answering such questions, additional information should always be provided. Keep in mind too, some of these you may define with the developer at the beginning of the project as well. Even if you have these in a basic format, you should bring them.
From these lists you should get a good idea of what almost any creative project might from you. These are important as making sure you have these before starting the project will only make the project go faster and avoid issues and delays. These will help you communicate clearly the idea you have to your developer, so you get the results you want. Many firms, like mine, have a process that helps clients define the project clearly early in the process. This helps make sure you don’t miss anything and ensures the firm has everything they need. Every firm and developer is different too, so some of these may be better than others and some may require something completely different.