Although Sills' voice type was characterized as a "lyric coloratura", she took a number of heavier spinto and dramatic coloratura roles more associated with heavier voices as she grew older, including Bellini's "Norma", Donizetti's "Lucrezia Borgia" (with Susanne Marsee as Orsini) and the latter composer's "Three Queens", "Anna Bolena", "Maria Stuarda" and Elisabetta in "Roberto Devereux" (opposite Plácido Domingo in the title part). She was admired in those roles for transcending the lightness of her voice with dramatic interpretation, although it may have come at a cost: Sills later commented that "Roberto Devereux" shortened her career by at least four years.