Nothing more than it needs to be, and none the worse for it.
Pacific Rim is a very solid movie. The setting is established nice and succinctly with a sequence and narration of how things got how they are in the film. After the narration is over, the first action scene is mere minutes away. The CGI and effects are fantastic, which makes all the fights a joy to watch.
The plot your basic underdog story; humans face insurmountable odds but persist anyway, and eventually come out on top. It's nothing special, and the film knows this and doesn't attempt to hide it. It doesn't pretend it's deeper than it is, which only works in the movie's favor. It's a very honest movie.
This doesn't mean that there's no character interaction, however, but the characters are good and enjoyable; Raleigh and Mako have good chemistry and play off of each other very well, as do Raleigh and Pentecost. As a result, the character scenes are never boring, nor do they last for too long, so you never feel like you're waiting for another fight scene.
Perhaps it's biggest flaw is that the other Jaegers don't get as much screentime as Gipsy Ranger or Striker Eureka. Crimson Typhoon is dispatched at one point in the film and is quickly taken down. When Cherno Alpha comes to their aid, they too, are quickly finished off.
All in all, a very good film to just sit back and enjoy with a bag of popcorn by your side. I recommend anyone reading this to go see it if they haven't already.