Although the movie mostly follows the story of Will Hunting, it involves tons of scenes where Will isn't present or even aware that the scene is happening. This goes especially for the scenes where Sean Maguire and Professor Lambeau fight about the different visions they have for Will's future.
We also get a chance to see things in this movie from a bunch of different angles because we know things that individual characters don't. For example, Professor Lambeau at one point has no clue that Will's friend Chuckie has been going to meetings for Will and scamming money from Will's potential employers. This is all pretty funny to begin with, but it's even funnier for us audience members who get to know everything that's going on… even when some characters don't.