There's a large and important difference between eliciting requirements and inventing them. It's rare for all of the requirements to be clear and neatly arranged, just waiting to be harvested; on the other hand, it's dangerous to invent requirements with insufficient evidence and/or input from the stakeholders. What if you end up with requirements that don't actually meet the business need? Better to avoid the embarrassment by creating a plan for eliciting the requirements carefully and accurately in the first place.
Eliciting the requirements nearly always requires that you extract the business needs from the stakeholder, the documentation, etc., and brainstorm the various options for meeting the needs and supporting the solution. Avoid gambling with the project's very foundations by "guesstimating" the requirements. Instead, capture the requirements and document them carefully, and follow up with an iterative process to continually review and refine them. A formal elicitation process is the foundation upon which every successful project is built. It will empower you to draw up the correct set of requirements, and help ensure that the project proceeds smoothly and your stakeholders remain happy.