Trevin Wax: How would you respond to the person who says the forms of worship are interchangeable, but the message must always remain constant? While admitting there is flexibility in forms from culture to culture, I think you'd want to push against the idea that the forms don't matter.
James K. A. Smith: Absolutely. I think we buy into a form/content distinction precisely because we've reduced the Gospel to a "message." So then we think we can just distill that "message" (the content) and then drop it into any "form" we want.
But as I argue in the book, forms are not neutral. Indeed, that was one of the core arguments of the first volume, Desiring the Kingdom: cultural practices that we might think are "neutral" - just something that we do - are actually doing something to us. They are formative. But what they form is our heart-habits, our loves and longings that, as we've already mentioned, actually drive our action and behavior.
So you can't just go pick some "popular" cultural form and insert the Gospel "message" and think you have thereby come up with "relevant" worship. Because it's more likely that you've just imported a secular liturgy into Christian worship. Sure, you might have changed the content, but the very form of the practice is training us to love some other vision of the good life. This is why I think a lot of innovation in worship, while well-intentioned, actually ends up welcoming Trojan horses into the sanctuary.
The response is not to come up with "the next best thing" in worship. It is to find new appreciation for historic Christian wisdom about the form of worship for the sake of discipleship.