The two leads' chemistry is critical to this movie
The way they feed off each other is fantastic, with the riffs and casual conversation. You don't see a lot of that anymore. If it was any two other actors this wouldn't have worked nearly as well.
Another thing I'm noticing is how great Danny Glover is in this movie. Really under-appreciated performance. I think some of his dialogue must have been ad-libbed. "Cause I shot him in the leg. Now I'm happy. I'm gonna go over here and be happy." There are parts where he is obviously doing it, like when he introduces Riggs to his family. It is so natural and allows the characters to feel realistic. Love it.