Composers use music notation programs throughout their creative process. Those programs are essentially elaborate structured document editors that enable composers to create high-quality scores by enforcing musical notation rules. They effectively support music engraving, but impede the more creative stages in the composition process because of their lack of flexibility. Composers thus often combine these desktop tools with other mediums such as paper. Interactive surfaces that support pen and touch input have the potential to address the tension between the contradicting needs for structure and flexibility. We interview nine professional composers. We report insights about their thought process and creative intentions, and rely on the ``Cognitive Dimensions of Notations'' framework to capture the frictions they experience when materializing those intentions on a score. We then discuss how interactive surfaces could increase flexibility by temporarily breaking the structure when manipulating the notation.