"Frances the Mute" featured the largest array of guest musicians on any Mars Volta album to date. Flea, who played bass on "De-Loused in the Comatorium", this time contributed trumpet to "The Widow" and "Miranda...". "L'Via L'Viaquez" featured guitar solos from John Frusciante as well as Omar's childhood hero, salsa pianist Larry Harlow, while "Cassandra Gemini" had Adrián Terrazas-González (who eventually became a full-time member of the band) on woodwinds. All the tracks also included full string and horn sections, arranged by David Campbell with the help of Omar Rodríguez-López.