Service projects are generally defined by the scope of work. You need to understand the scope to be able to estimate the size of the project and determine what to provide. You need to know enough about the scope to formulate your approach to performing the work required. The scope of work will be based on the customer's deliverables and performance requirements, so you need to make sure you understand what they are. If you do not, you should ask.
While the scope of work is critical for figuring out what to propose, it may be a secondary consideration for determining whether your proposal is selected. The first thing you need to know is how to avoid being disqualified. Does the customer have formatting or submission requirements, or are you on your own? Once you know how to format and present the proposal without being disqualified, you need to understand what criteria the customer will consider in evaluating your proposal, and what process they will follow in evaluation. You should find out as much as you can about the customer's preferences at all levels, including technical, staffing, and management. Finally, you should make sure you understand the customer's goals, including long term, short term, and project specific goals.
Your goal in gathering this information should be to determine how to position yourself against the customer's evaluation criteria, and to provide them with the reasons why they should select you. Along the way, you need to gather enough information about the competitive environment to know how to position yourself against the competition. This is critical for a winning proposal. All the information that you gather before writing your proposal should support your understanding of these items.
Your proposal will need to address the approaches that you will take in fulfilling what you propose. Understanding the scope of work will help you estimate the number of people that it will take to do the work. But you will also have to research where the project staff will come from and whether they will be transferred internally, recruited, or incumbent staffing that you will hire. The most important position will, of course, be the Project Manager, who is often critical for service projects. For each position, you will need to understand the qualifications required and which staff you will name or provide resumes for.
You should also be able to articulate your approach to managing the project, including how staff will be organized and how effort/hours will be allocated. If there are multiple locations, you should address how they will be managed. If there is an existing effort, you should be able to address how you will manage the transition from the current environment to what you are proposing. For complex projects, you may need to be able to provide a work breakdown structure (WBS). You should also prepare a schedule of deliverables. The WBS, allocation of effort/hours, and schedule of deliverables can serve to validate each other and help to ensure that you have the right estimates to base your pricing on.
In assessing the scope of work, you should evaluate whether you can perform the work on your own or whether you will need subcontractors. In assessing the evaluation criteria, you may find that having certain subcontractors can increase your chances of winning, even if you could do the work without them. For service projects, this is often based on whether you have the right project experience or references to win.
When you speak to a customer about a potential opportunity, you should be trying to develop an understanding of the topics described above. If you can talk to the customer, ask them questions until you have a full understanding. If you have a Request For Proposal and can't answer these questions, you will probably lose to someone who has the answers. If you have never talked to the customer, then how can they believe that you have sufficient understanding of these topics to keep them satisfied? Put yourself in the customer's shoes --- if you have to bring someone in to do work for you, you want them to ask the right questions to do a good job. So don't be timid about asking questions --- you are demonstrating how you will take care of them after you win the proposal.