This was originally a part of my question about partnering with a website, but I think it's separate enough to deserve its own post:
Many books are published with an introduction by a better-known author, or academic, or public figure, whose name on the cover helps sell the book. I assume in most cases all the arrangements are handled by the publisher. But what if you want to or are able to arrange the introduction yourself? Is the introducer a co-author? Do they get a percentage of royalties? Are they paid a flat fee? Or do they typically do it out of the goodness of their hearts?