Also an awkwardly intimate situation from Mark's life? Sitting behind a two-way mirror while people judged and criticized him.
The process is known as "testing" in the entertainment industry, and is used to get honest opinions about a new show. Since Mark happens to be an Executive Producer as well as the star of 9JKL, he went to watch—and it may have felt a bit personal!
He shared what it was like during a visit to The Late Show, and Stephen was able to commiserate.
WATCH: Mark Feuerstein Answers: Is 9JKL Based On Real Events?
The two also spoke about the influence of the legendary comedian Steve Martin, who was also a guest on Thursday's show.
READ: Get Closer To 9JKL Star Mark Feuerstein... But Not That Close
Watch all-new episodes of The Late Show with Stephen Colbert weeknights at 11:35/10:35c on CBS and CBS All Access.
Watch the series premiere of 9JKL on Monday, Oct. 2 at 8:30/7:30c on CBS and CBS All Access.