Theodore Twombly, played by Joaquin Phoenix, is a letter writer. That's his job. He writes letters for people to other people for a company called (something like) Hand-written Letters. This letter writing that he does is, in many ways, a metaphor for the entire movie as the letters are personal letters, sometimes love letters, that he's hired to write (and, in some cases, has been writing for the same people for years and years and knows them extremely well), but he dictates the letters to his computer and they are printed out to look as if they are written by hand.
It's unclear within the context of the movie whether the people receiving the letters know they are written by a third party, but I have a hard time believing that they wouldn't know this since the whole letter writing thing is a "thing." Basically, Theodore facilitates other people's relationships by filling these letters with, what I'll call, manufactured emotions, but he can't maintain his own relationships, which is something we find out at the very beginning of the movie, so no spoiler there. He's in the middle of a divorce with his wife and is distant from his friends.
Now, there will be spoiler issues in the rest of this.
With all of that in mind, it is no surprise that Theodore easily finds himself slipping into a relationship with his new Operating System, the first OS with artificial intelligence. As his wife accuses him later, he can't do relationships with people that are right there in front of him. It's probably why he's so good at the letter writing. And he is good at the letter writing, one of the best, at least. It's no surprise that he finds himself attracted to this physically distant intelligence.
But it does open the door to exploring the idea of "what is a relationship?" What is required for a relationship to be legitimate? Is his love real? Is Samantha's (the OS)? Does she need a body in order to manifest the relationship? And it's not just him, because we get pieces of information in the movie that other people in society are dealing with the same struggles. Or the opposite struggles, as we learn that at least one user has a real hate relationship with his OS.
The movie doesn't really try to answer these questions, which is good. It just shows us that the questions are there and should be considered. Although there are a few concrete answers, one of which is that, at least sometimes, we do need the physical presence of another person. Especially in dealing with loss.
Joaquin Phoenix was great as Theodore. It's a very subdued performance, because Theodore is a very subdued individual. I think performances like this get overlooked because they're not outrageous, but Phoenix is much more believable in his role than, say, DiCaprio as Belfort, which is not to take away from DiCaprio's performance, but I think it's easy to look at a movie like The Wolf of Wall Street and think, "Wow, he was amazing" but forget about roles like Theodore Twombly which rely on conveying emotion rather than exaggerated action. Phoenix was superb at conveying the emotion of Twombly.
Even more amazing, though, was Scarlett Johansson. Everything she did was conveyed through voice only, and it was incredible. I don't think I've really given her a lot of credit in the past for her acting ability, which is not to say that I didn't think she was good; I just didn't think she was, well, better. It's too easy with her to think, "Oh, she got this part because of her looks," and not really credit her for the actual acting. But she's not physically in this movie, and what she did with her voice shows that she is better. She didn't even have the help of animators to convey her emotion; she just had to bring it audibly, and she did. It is actually upsetting to me that she has been dismissed from the Oscar nominations because she wasn't physically in the movie. That's just wrong.
Her is a great movie. It's a thoughtful movie. There are no explosions or car chases or alien invasions. It's sci-fi that could happen. And, yes, it was interesting... but in a good way.
