At this point, I assume this show gets the caliber of guest actors it does because it is very good, and actors know they will have at minimum a decent scene to play. At a minimum.

In addition to everything else, I thought this particular episode was ridiculously funny. They have a deep bench of characters and do a great job finding humor (both low and high) without trivializing or undermining the seriousness of the show. (unlike David E. Kelley.)