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Pro Skater Jason Dill played Len, the guy getting tattooed by everyone at the party in S1 E4. Also, Ed Templeton is shown breifly at the party in that same episode. Jason Dill's character shows up in season 2 as well.
The long flight of stairs Mickey and Gus walk up in S1:E2 is the same one that Laurel and Hardy carried the piano up in The Music Box (1932).
Iris Apatow, who plays Arya, is the daughter of Judd Apatow, the executive producer of the show.
Gillian Jacobs plays Mickey Dobbs who suffers with alcoholism on the show. In reality, Gillian is a teetotaler and practices complete personal abstinence from alcoholic beverages and drugs.
Stella (Jessie Ennis) originally had a bigger part in season three. However, Ennis suffered a leg injury and needed to rehabilitate; her episode count was dropped to three and the writers and crew worked around her injury, as she was still not completely healed by the time filming began.
Gus's Springwood apartment is actually Oakwood apartments in Studio City, CA.