Complex sentence: Many Hollywood stars gave brief speeches or performed acts, rehearsed by Kay Thompson and directed by Roger Edens, and stayed at the Statler-Hilton Hotel where preparations and rehearsals were photographed by Phil Stern.
Split into simpler sentences: Many Hollywood stars gave brief speeches or performed acts, who were rehearsed by Kay Thompson and directed by Roger Edens. Stars, producers, directors, and other show personnel stayed at the Statler-Hilton Hotel, whose preparations and rehearsals were documented and photographed by Phil Stern.