"No, writing musicals is the hardest thing in the world. And it was really funny, because I remember when the South Park movie came out, there were some critics that said, 'Well it's obvious that in order to get it to be 90 minutes they filled some time with music.'"

In this quote, Trey Parker is revealing his belief that writing musicals is an extremely uphill struggle. He remembers a time when the South Park film was released and some critics recommended that the music was simply utilized as filler to reach the desired 90-minute runtime. Parker discovers this idea entertaining, as it undermines the enormous effort and skill that enters into developing a successful musical. By stating that composing musicals is the hardest thing on the planet, Parker highlights the difficulties and complexities involved in this art type. He is likely defending making use of music in storytelling and highlighting the value of music in producing a cohesive and impactful story.