Earlier quoted context omitted.
Their past performance is a good indicator of their commitment. I am also a firm believer that "environment trumps discipline." So maybe (and just maybe) their inconsistency with your project is because they've failed to create an environment that forces them to work. (Thats why many lazy folks become disciplined if they're given a deadline with a promise of a bonus if they meet the deadline.) I do rely on a few tric…
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You can ask them to start working with you on a small part of your idea for which you want them to partner with you too.
But I think a better idea is to create a new small project. And ask them to work on it for you first. Keeps the real project a bit distanced before you both are ready to commit to a partnership.