One of the things I like about writing biography is that it is both a narrative and interpretive art. With a biography, we have a life story to tell, with a beginning, middle, and end, with intersecting characters who have inner and outer lives. But we also have some explaining to do—a duty to explain our subject in the context of the surrounding world, and to explain that world through the life of our subject.
That brings us up against one of the oldest, and most useful, pieces of writing advice: Show, don't tell. If you can unfold your explanations and interpretations through the actions and speech of your characters, so much the better. You avoid halting the narrative flow, or deadening the reading experience. For example, in writing about the famous Erie War, the fight for the Erie Railway between Cornelius Vanderbilt and Daniel Drew, Jay Gould, and Jim Fisk, I illustrated some of the larger significance of the conflict by quoting newspaper editorials, which also illustrated how closely the public was watching these events.
But it's worth bearing in mind another piece of advice, from George Orwell. In "Politics and the English Language," he sets out some very fine rules for writing. The last rule is to ignore all the previous ones, rather than say something barbarous. The point, of course, is that all rules for writing are mere guidelines. With the right reasons, and the right skill, you can and should break them.
So it is with "show, don't tell." Sometimes a biographer really should tell—explain what's going on, why it matters, how it fits into the larger context of the times, or the main character's life. This is especially true because a good biographer shouldn't take existing interpretations of the times (and main subject) for granted. A biographer is historian as well as writer, and should think hard about the context, and come up with new conclusions when warranted.
How to tell, without harpooning your narrative? There's a number of tricks to it. First, write well. Second, make use of the reader's primary motivation: expectations. My favorite definition of plot is the creation of expectations, followed by their fulfillment (though not always in the way the reader expects). Before launching into an interpretive passage, it helps to set the reader up for the action to follow. Before a discussion of gravity, it's good to stop the narrative right after Wile E. Coyote runs off the edge of the cliff and looks down, feet still pumping, but before he plunges.
Third, interpretive passages have to feel like an organic part of the narrative. The reader must feel that he or she needs to understand what you're explaining in order to follow the events. This is a good test for the writer, because if the reader really doesn't need to know, then the passage should be moved, rewritten, or cut entirely. Another example from my forthcoming book (which I hope works the way I've described): When Vanderbilt challenged the steamboat monopoly on the Hudson River, he appealed for public support in terms that directly reflected the politics of the day. I felt that I had to explain politics (and offer my own interpretation of them) to fully explain what was at a stake in this business battle, why it mattered to the public, and what it said about the times.
Still, a biography remains a story, first and foremost. I find that historical themes draw me to a subject, but a biography of that subject is not an academic monograph. It is, we hope, a literary product, one that gives the reader pleasure—and a reason to keep reading—from the first page to the last.