This is such an awesome scene, the fact that Chuck wouldn't sell out a friend even though he might lose the most important thing in his life says a lot about his character. Casey has always secretly been a Charah shipper
Can we take a moment to appreciate that Casey completely stuck his neck on the line to tell Sarah this? He outright admitted first-degree murder to a government agent; she would've been well within her rights to arrest him on the spot. Of course she never would've done but still - the little lines of trust that had developed between them by this point, whereby he could tell her this info without fear of reprisal, were fantastic.
This scene was really well done. The "unlike us" is perfect. It exposes her hypocrisy and makes the viewer feel some pain for Chuck, since he did it all for her.
Ever notice how casey is the one to give the sage advice when its the most critical?
There is this scene
2) The scene where he tells chuck it doesnt matter where you propose and he tells where he proposed to his wife which wasnt in a glamorous setting
3) when he gives sarah the whole report of the operation in season 5 where she says she loves chuck when she was going to leave and it forces her to stay.
This scene is like a ray of light coming out of the darkest moments of Chuck and Sarah relationship in the first half of Season 3. Such a big relief for the fans holding their breath in those episodes.
Casey said "You have a nice life Walker" because he realized that she was leaving with chuck even before he came clean on killing the mole. Casey admitting that chuck didn't do it just put Sarah's mind at ease and that she is making the right choice
Chuck risking his own life to save Shaw is what made Sarah realize he hadn't changed, after all. Plus, him actually saying the words that he loved her, even though she obviously knew already, is what really allowed her to tell him, as well. She goads him into it, during their "first real date" ("What about me?"), and when she tells him she doesn't have anybody in her life who cares about her. So she obviously likes hearing it. I'd even argue she does this with her previous statement about having "a type." On some level, Sarah must realize that Chuck and Bryce are quite similar. It's why it was so hard on her every time Bryce would show up.
I know the central relationship of the show was Chuck and Sarah, but seeing John Casey change due to loving Chuck and Sarah, and secretly rooting for their relationship was one of the best facets of this show.
That is why this tv show was so good. When you watch the scene and at the end your heart beats 3 times faster you know that someone done his job well. Chuck had plenty scenes like this, scenes that I can recall from my memory anytime, 10 years have passed and those scenes are clear to me as if it were yesterday.