Did you recognize the actress playing the "patient / mother"? You know her for example from Hotel Del Luna (where she played the roomkeeper, one of the 3 ghosts working for IUs character). Also: the funny assistant played in an episode of Hotel Del Luna as guest star and Gang-tae himself was the big cameo at the very end (Hotel Blue Moon). That cameo in Hotel Del Luna was his first official appearance in a tv show after his military service and a big thing (because he already was a big star before his military service). After that cameo in Hotel Del Luna he started filming Its Okay To Not Be Ok.