Complex sentence: Sondheim wrote the lyrics and music over a two-day period during rehearsals for the play's Broadway debut, specifically for the actress Glynis Johns, who created the role of Desirée.
Split into simpler sentences: Sondheim wrote the lyrics and music over a two-day period during rehearsals for the play's Broadway debut. He created the song specifically for the actress, Glynis Johns, who created the role of Desirée.