I'm going to try to keep this from becoming just a Meryl Streep and Stanley Tucci love fest, and actually take some good writing lessons from the movie, but I want to just start out by saying how incredibly talented and stunningly superb both Streep and Tucci are. They are both so believable and, in this movie, utterly adorable. I could very easily go on, but let's look at some specifics and see what we can learn.
Adaptability: If you look at the last movie Streep and Tucci did together (Devil Wears Prada, another amazing movie), you see the stunning adaptability both of them show in going from the characters in that movie to the new ones. Particularly Streep. She went from being one of the most believable snarky witch bosses ever on screen to a very successful portrayal of a sweet, vulnerable, real life woman. Within a book, you've got characters as diverse as those ones that have to come from your own head. Look to Streep's performances as a brilliant example of how its done. And this is applicable not only for character, but genre. If you've never written speculative fiction, get out of your comfort zone and try some. Maybe try a screenplay or poetry, if that's something you haven't tried before. It may turn out to be fun, and will most likely improve your writing in whatever genre you typically write in.
Vulnerability: The lesson from this is that there absolutely must be conflict. In this movie, the Childs' marriage is practically perfect, and provides some wonderfully poignant and passionate moments. The conflict is not between the two of them. But then we get these scenes of vulnerability, where we see how unsatisfied he is with is job, and how she wants a baby so badly she sometimes just can't take it. These moments show how much the two rely on each other for emotional strength and support, and makes the happy moments in their marriage mean so much more. So if you want a sweet, poignant romance in your book, that is wonderful and will touch many readers. But the trick is to make the characters vulnerable in other ways, to show how much the happy relationship is needed. Every character must be vulnerable in some way so we can relate to them.
Balance: Streep way out-performed Amy Adams, not just in talent but in story. Make sure that if you have two leads you either deliberately make one better then the other, or make them equal. Even with antagonist and protagonist, you can't make one less powerful or interesting then the other. They need to be deliberately matched, or else its unbalanced, which, if thats what you're going for, then great. I think this is just something to be aware of so we don't misuse it.
Hope this helps! I highly recommend this movie/Meryl Streep and Stanley Tucci. Learn from it, but also, just enjoy it!
In the words of Julia Childs, Bon Appetit!