If the reluctant but capable participant is a 'guru' type who is in great demand (technical experts often fit this profile), make a point of building a longstanding relationship - one that goes beyond the current project. You'll probably need his/her input for other projects and, since they're typically in high demand most of the time, you'd be wise to form a strong alliance.
Focus on learning what the reluctant/capable participant wants, needs and likes, both about the project and in general. The chances are pretty good that you won't have much that s/he can't get from work on other projects, but it never hurts to hunt for unique benefits.
In the event you don't have unique benefits for the reluctant/capable participant, focus on the benefits that the organization will derive from the project.
If the reluctant/capable participant is prone to extensive and somewhat technical explanations, take the time to listen; really LISTEN. Ask good questions and devote some time of your own to learning more about their area of expertise. You'll demonstrate your valuing of the person and you'll learn something in the bargain.
When/if you get a commitment, even a tentative one, ask the reluctant/capable participant to put it in his/her planner. The commitment is more likely to stick if they write it down than if they don't.