Writing real people

My alternate history series is starting to take shape in my mind. I'm picturing scenes, getting a sense of how my characters will interact, etc. I love this part of the process because it's when creativity goes into warp speed, and my body is folding laundry or buying groceries, but my mind is 200 years and almost 5,000 miles away.

None of my previous books have featured real historical figures as anything more than names tossed in to show readers I've done my homework. The Sergeant's Lady refers to Wellington and to Robert Craufurd (one of Wellington's subordinate generals, commander of the Light Division), but neither gets any screentime. And that works fine when your protagonists are a sergeant and a captain's widow. But half the fun of an alternate history is taking real people and putting them in the altered circumstances.

So I'm developing a few of my own characters, including my main protagonist and his love interest, and researching certain key real people, at least one of whom is nearly as important as the protagonist. And it's WEIRD having so much about a major character be determined before I've written a single word. He's in his 30's when my world noticeably diverges from the real world, so he's got a huge chunk of backstory that's right there in the historical record. His personality is largely set--I can justify certain changes based on how I think he'd respond to my scenario, but I've got to work with the material reality gave me. I feel like I owe it to the real man to try to imagine what he would've done, and not to just make him do whatever is convenient for the story or make him a mouthpiece for my worldview.

I don't mind the restrictions, especially because it IS alternate history. But I don't think I could write the sort of fictionalized biographies Philippa Gregory, etc. specialize in. I'd feel too boxed in by reality.