When writing nonfiction, "dialogue" can be a dangerous word. My first drafts of Boys of Steel did not contain any.
Librarians cast doubt on books which claim to be nonfiction but which don't source quotations they include. If any of its dialogue is made up, a book is (obviously) not pure nonfiction.
Once I discovered a picture book about a celebrated poet which included lots of dialogue. I asked the author about that. He said the words were a combination of excerpts from the poet's autobiography and some things the author "rather assumed." For that, he told me, the book got "whacked in a couple of reviews."
Get a review that questions your authenticity and you run a real risk of losing library sales.
That is why, when an editor read an early draft of Boys of Steel and suggested I liven it up with dialogue, I balked at first.
However, I grew curious. I went back through the Jerry and Joe interviews I'd used as source material and found instances where I could replace exposition with a quotation. As luck had it, these happened to space out fairly evenly throughout the manuscript. In one or two cases, I had to change a verb tense, but otherwise, I was quoting verbatim.
This was the version that sold. However, at one point my editor said she doesn't feel picture book biographies need dialogue. I told her I didn't, either, but another editor had encouraged me to try it and I liked the outcome. Later, I asked my editor if we could add this to the acknowledgments page: "All dialogue is excerpted from interviews with Jerry and Joe." She didn't feel that was necessary, either. Referencing my fellow author's misfortune above, I pushed for it. My editor kindly obliged me.
The very first review of the book singled that out: "A bibliography and assurances that 'all dialogue [was] excerpted from interviews' puts factual muscle behind the subject’s literary brawn." My editor joked that now I can blog about how she resisted that at first. One factor that makes her an exceptional editor is that she respects her writers' instincts—and another is that she has the style even to mention this.
I've always been careful with how I use and cite information in my work. Absolute truth may be unknowable but writers owe it to readers to come as close as possible.
9/8/14 addendum: what can happen if you do not cite sources and other dangers of writing dialogue (an article I wrote for The Horn Book).