Task: Question Generation
Input (English): Passage: Impresario and author John Hollingshead, the lessee of London's Gaiety Theatre since 1868, had produced a number of successful musical burlesques and operettas there. Indeed, Hollingshead "boasted that he kept alight 'the sacred lamp of burlesque.'" Gilbert and Sullivan were each well acquainted with the Gaiety and its house artistes.  Gilbert's Robert the Devil (a burlesque of the opera Robert le Diable) had been on the programme on the theatre's opening night on 21 December 1868, with Nellie Farren in the title role, and played successfully for over 100 nights. Constance Loseby and Annie Tremaine (both of whom had roles in Thespis) were also in the cast of Robert, and Arthur Sullivan was in the audience on that opening night as one of Hollingshead's guests. It was a great success, "received with a storm of approbation". Less successfully, Gilbert had also written a play for the theatre in 1869 called An Old Score. Hollingshead would later say that the piece was "too true to nature". By late September or early October 1871, Gaiety programmes announced that "The Christmas Operatic Extravaganza will be written by W. S. Gilbert, with original music by Arthur Sullivan." There would be prominent roles for the popular comedian J. L. Toole, as well as Farren, the theatre's star "principal boy" in all of its burlesques.
How and when the pair came to collaborate on Thespis is uncertain. Gilbert was a logical choice for the assignment. With seven operas and plays premièring that year and over a dozen other burlesques, farces and extravaganzas under his belt, he was well known to London theatregoers as a comic dramatist. Sullivan, however, was at this point mainly known for his serious music. His completed music that year included the choral cantata On Shore and Sea, a suite of incidental music for Shakespeare's The Merchant of Venice, and numerous hymns, including "Onward, Christian Soldiers". He did have two comic operas to his credit, Cox and Box (1866) and The Contrabandista (1867), but the latter was four years in the past and had been unsuccessful. In September 1871, Sullivan had been engaged to conduct at The Royal National Opera, but it failed abruptly, leaving him unexpectedly without commitments. Hollingshead's offer of a role to his brother, Fred Sullivan, may have encouraged him to write the music for Thespis.The production "aroused a great deal of interest and speculation". Ironically, it had "probably the largest audience" of any Gilbert and Sullivan première, as the Gaiety was the largest of the five London theatres at which their joint works premièred.
Output (English): What are the full names of the two individuals for whom the how and why of their collaboration on Thespis is unknown?