West Side Story is ground-breaking in a lot of ways, but when it comes to delivering the information, it tends to rely on the tried-and-true technique of moving to the character we need to see when we need to see him or her, then moving back as the story demands. It's a more-or-less straight linear narrative.
Having said that, it still pulls off the odd trick or two that deviates from our expectations. The opening dance number is a big part of it. The dances are intended to convey several months' worth of plot in a few minutes. From the way the gangs watch and follow each other so closely, you can tell that these rivalries have been a long time in the making. A more traditional film might show us that in a montage or something a little more typical (read: boring). Instead, it uses the musical's inherent ability to defy reality (we don't usually see people breaking into elaborate dance numbers on the street, even in the era of flash mobs) to push the story forward.
Something very different happens when Tony and Maria first meet. The dance hall falls away. Everything gets blurry and the sounds fade. Tony and Maria see only each other. Again, the movie uses the musical number to help convey that: suddenly we're not in anything resembling reality, but one a dark stage with couples moving in perfect sync with each other.
It's not realistic at all, but it helps us understand what Tony and Maria are feeling at that precise moment—that floating feeling, that bolt from the blue. The pace slows; the music changes. It's an intimate moment that puts us right in the heads of those two.
The omniscient narrative style becomes mind-blowing spectacle during the "Tonight" quintet. The scene cuts back and forth between Jets, Sharks, Anita, Tony, and Maria—all happening at the same time, everyone singing their hearts out about what's about to happen tonight, each with their own take on what's about to go down. Besides letting us know what each is thinking, there's an incredible build-up of tension because we can just tell that things aren't going to end well. They're all heading for a disastrous confrontation.