The thing about Bridget Jones's Diary is that it isn't a diary. Not really. The chapters start out in diary format, but they transition to standard first-person narrative partway through, until the end of the chapter.
I wasn't as big a fan of the book as some people, but I did think that was an interesting technique -- a few paragraphs of diary format, and then segue into a standard first-person POV, dropping all the diary-style abbreviations, etc., until the next chapter, where again it would start in diary format and then segue out of it. It was fairly seamless, at least to someone who wasn't a writer, analyzing the writing, but if you look at it, I think you can see where it switches in each chapter. For me, that helped with the suspension of disbelief, b/c it wasn't really diary format for most of the book.
JD