Each season, Girls has churns out provocative, sometimes infuriating, story lines. Some episodes, like this season's "American Bitch," give rise to at least one think piece per pop culture publication unpacking the story's ins, outs, and broader implications.
As the four protagonists flail through their 20s and (maybe, finally) do a little growing up, they come in contact with many other characters who ultimately wind up reorienting how the girls see themselves and their place in the world.
In this way, Girls' structure has provided ample room for guest appearances from cultural icons to up-and-coming actors alike. So, for American actors, appearing on Girls is almost like British actors appearing in a Harry Potter film. It means you've made it, or will, pretty soon.
When Girls is over, we'll look back to these celebrity cameos with a wink.