If you haven't yet caught the captivating
Though its star,
If you step back and consider the premise, it gives you a deeper appreciation for what Phoenix managed. Nearly all of his scenes are spent as the only human on screen, and he's usually having a conversation with a computer, more specifically an operating system named Samantha. Sam is brilliantly voiced by Scarlett Johansson, but for now focus on Phoenix. His challenge was to create a plausible and engaging emotional life for Theodore Twombly, a man whose job is writing Hallmark-esque personal letters for other people.
The actor's performance is a master class in subtlety. Slightly less animated and Theodore would have bored us to death; slightly more and he would have become a silly fool. Instead, Phoenix makes the romance between a guy and the computer-constructed essence of a girl achingly real.
Jonze is equally astute in using Theodore and Sam to contemplate our growing interdependence on technology, finding the sweet spot between erudite and insufferable.