R.S. has good ideas there; I recommend the videos.
In regard to nonfiction: unlike fiction and memoir, you're not expected to have the book complete already. That said, putting the proposal together will take some time. There will likely be other books in your field already -- which tells agents that there's a viable market -- and they may want you to revise, cut, or add elements. That's all ok. Think of your proposal as a business proposal, because that's what it is. You're looking for a business partner.
The proposal helps an agent and/or a publisher assess your thinking process as well as your knowledge of your field. Sample chapters will be a better way than a query (which is also part of the proposal) to assess your writing ability and voice. As RS said, don't sweat the likelihood of changes. Your book will almost certainly leave a lot on the cutting room floor, but those scraps might also be useable for ancillary projects. One chief difference between nonfiction and fiction is you're expected to be a bit of an expert in nonfiction. The agent will want to know why YOU should be the author of this book. Having a great idea is fine, but they're a dime a dozen. Having an author who's recognized as an expert in the field carry that idea over the threshold is what can sell books.
I recommend conducting those interviews and writing a couple chapters as part of the proposal. If those end up being unused, then adapt them to your blog or website or as something to include later as an incentive to potential readers later. You can make an unpublished interview into a giveaway when readers sign up for your blog, for example.