Don't Watch This! Anne Hathaway's face and no more...

The book or the film? The book then the film? The film then the book? Which one do you prefer? Or is the question better put 'which one is less bad'?
To me, good books and films don't combine at all well. Books where the characters think a lot, where they feel a lot. These don't make good films. Too much of it gets lost.
Of course, films where characters run around a lot, and squirt blood out of themselves and fall over, these make bad books. Or possibly don't make books at all.
'One day', and here is the quote, is one of the best books I've read about modern relationships among children of the seventies and eighties. (Not actually that modern, these days).
The characterisation is pitch perfect. The dialogue is nailed on. The characters, again, are so realistic that you can imagine them, perfectly, sitting with you and your friends in a pub on a Sunday afternoon.
Dex is possibly less sympathetic than Em, generally, though they both have their moments. But that doesn't matter, they can't both be completely sympathetic, they wouldn't be as completely real as they are.
One of the bestsellers of recent years, I'm sure you know how the novel works. It tells the story of every July 27th for twenty years, in the life of Emma and Dexter. It's about life, and friendship, and growing up. If you're one of the few people who hasn't read it, get a move one. It's brilliant.
But don't see the film. Don't even watch the clip. It'll force the ideas and images of strangers - directors and cameramen onto you, hijacking your chance to invent characters faces and clothes, or the way they move.
To be honest, there are some things you can do nothing about. I knew Anne Hathaway played Em before I read the book. I couldn't help myself, I gave Em Anne Hathway's face.
Fortunately that's not a bad thing. I can live with imagining Anne Hathaway's face while reading a book, but my recommendation is that you leave it at that.
Anne Hathaway's face and no more.