Here are the situations regarding two people I know:

1. Person 1 is terribly bored by the work he does in radiology, but he’s extremely good at it, and organizations like product companies, health systems, and providers seek out his consulting services.
2. Person 2 is extremely excited and passionate about behavioral economics and wants to start a consulting firm. Unfortunately, this person has no experience in the area and can’t figure out ways to both find and close deals with clients.

To start a consulting firm, what matters is being able find clients and close contracts with clients. To stay in consulting for a sustained period, one also needs to be able to deliver the services. Passion (and expertise for that matter) can play a supportive role in all of these areas; it can make it easier for you to find clients, close contracts, and keep delivering quality services. I don’t believe passion is needed to get the ball rolling though. It might be something one should think about before starting to roll the ball, but business development is a must-know-thing for running one’s own consulting firm.