How do they feel? Are they satisfied? Are they happy? Are they accomplished? Do they think they deserve it? Do they ever did or feel the need to sacrifice their artistic vision for the audience and business? If they did do they feel guilty or feeling like a sellout? Do they ever feel wanting to quit, but cannot because of the audience or pressure from the business?
Besides my personal curiosity, I'd probably ask tips about marketing, attracting audience, fitting to audience's taste, and having confidence in their work. Marketing can hype up an otherwise mediocre work, so it's a huge factor in my opinion.