You can't give an answer unless you know what the question is. Most of the time we think we have something figured out but if you don't truly understand what is happening that has led to problem your counsel will be misplaced. This is especially true when you are first working with a client and you don't have the "inside baseball" of what happens day to day or have a true baseline of what is happening in their market.
One of the best tools to use when you're figuring all that out is a situational analysis. The best ones take into account the macro environment - what is happening in the market and with the competition that is creating external pressure on where a business is - as well as the micro/internal environment that is adding to or taking away from a company's effectiveness. The great thing about marketing is that we have to take into account all of that when determining solutions and programs and we have the opportunity to have a direct impact on both, which places even more importance on getting the situation right.
The simplest form of a situational analysis is SWOT (Strength/Weaknesses/Opportunities/Threats), but some engagements call for something beyond that in order to really get what's going on. A SWOT analysis based on a viewpoint of a couple of people at the client and a few Google searches can skip off the atmosphere of what is happening and doesn't get to the heart of the true issues at play. Last week I wrote about doing a broad range of client interviews to get a broader perspective on both in the internal and external forces. Industry reports and third party analysis also help provide independent perspective and can be used to validate the points you will make to your client. At the end of the day you need to make sure you have a strong understanding of what got your client to where they are and what the market and their competitors. Quality marketing solutions will only come from determining the right answers to the proper questions.
So regardless of how fancy or formal it is, make sure you have a good understanding of the external and internal forces that have led a client to come to you to fix their situation. Without it your proposed solutions will be off the mark and you won't be any closer to fixing what ails them.