Your role and perspective is important - owner with the idea, investor, consultant, partner, etc. There a couple of things that shouldn't be skimped on or skipped over during the "diligence" phase.
-- Be careful about "believing your own internal press" and work to understand and validate the going-into-business and go-to-market assumptions.
-- Develop your purpose and vision for the business, and why you want to be in it besides making more money. Is there an end game?
-- Go beyond market research and engage prospects and customers to understand real needs, perceptions, problems, and buyers motivations
-- Get the overall business models - revenue, sales, marketing, and customer experience and process figured out before investing heavily in product or service development
-- Resist the urge to make strategic decisions based on what the competition is doing (see comment on assumptions!)
-- Engage prospects early and often and don't hesitate to shift your solution based on their input/feedback.
I have seen quite a number of start ups focus solely on creating plans to get the funding, and fail to adequately flush out the customer/marketing/sales models and lose sight of why they went into business.
Do your market research. If you can, develop a prototype or develop a proposed menu of services & proposed price list, etc. and get feedback from the prospective customers you will be targeting.
Remember that people love it when you ask advice so really listen to these survey takers and be open minded. I have seen some start-up businesses get such enthusiastic feedback from this kind of market research that they created a customer list with pre-orders in one example, and got multiple free consulting offers & business referrals ($100Ks in sales) in another example.
Do your market research by talking to not only potential clients or customers, but also your competitors. Find out what they are doing right, what do you like about their services - and can you offer that too. Find out what they are doing wrong -or how you can provide the same service in a better or more efficient manner. This also allows you to make personal and business connections, in addition to allowing you to "test the waters".