This process of discovery and revision is, I suppose, what people mean when they talk about 'the characters taking over' during the writing of a novel. In my opinion, it isn't that they take over; it's that they develop their own integrity. And this feeling of integrity is an indicator (not always reliable, but usually so) that the decisions that you, the author, have made for your fictional creations are the right ones...looking back, I believe that if, instead of locating Merivel in the seventeenth century, wearing his ridiculous clothes and wig, I'd created a 1980s character--not a doctor, but a banker, say--made him a master of the universe and then brought him to a colossal fall, his story would have turned out to be as ephemeral as padded shoulder-wear and long ago vanished from the shelves. (Afterword, p. 401-402*)Restoration was even turned into a move (with Robert Downey, Jr.) and, according to Tremain, even became a stage show. Merivel, whatever you might think of him once you read the book, does deserve a chance to sparkle now and again.
In the course of the book, other characters label Merivel a Man of his Age. He loves the food and the clothes and the sport and the luxury of England with a restored monarchy. After twenty years of Puritan government, everyone wants to let their hair down. (Actually, they wanted to cover their hair with wigs but, you know, metaphor.) Merivel is the son of a glovemaker who heads off to Cambridge to study to be a doctor. But he's really a born hedonist. His talent at medicine frightens him because it constantly reminds him of the mortality and decay around him. During his wedding, when his father-in-law plays a beautiful song, all he can see--with his trained anatomist's eye--is the man's skull.
Merival, through a fluke, finds royal favor. Unfortunately for him, living in such luxury lets his hedonistic tendencies take over. Everything that was interesting and useful about him disappears. The king gets sick of him and banishes him to a manor in Norfolk until he learns to be useful. Merival, though, has to fall a lot farther before he learns to find a balance within himself. He sees a lot of tragedy working in a madhouse in the English fens, then treating people in London's Cheapside during the plague of 1665 and after the great fire of 1666. For the first almost half of the book, you start to get sick of Merivel yourself. But in the last half of the book, he is redeemed.
I liked that Tremain let her character breathe and be himself. Because of that decision, not only is Merivel a more rounded character, he's actually believable as a human being. There is no way that a man with as many flaws as Merival has can be completely transformed. To try to do that to him would have ruined the book and, as Tremain wrote, would have made it "ephemeral."
* Kindle edition.