I have only seen parts of that movie, so I'm guessing. If you have a long lens or zoom in, all the way, you will compress the crowd, so that they look closer to the subject than they really are. If they are not right on the focus plane, they will be out of focus.
If you have the shot and everything is in focus, you can bring the shot into After Effects. Make a Composition out of it and put it into your timeline. Now duplicate it. Now add the Fast Blur effect to the second layer, so that everything is blurred. Go to Layer 1 and use the mask tool to add points around your subject. When you close the last 2 points, that should let the blurry parts outside of the mask to come through.
Use Feathering to soften the edges of the mask. Click on Mask Shape's stopwatch icon and move the mask so it stays right with your subject.