Complex sentence: Robert Metzler, the show's business manager, believed that the incident had been planned in some way; during the dress rehearsal Niven had asked Metzler's wife to borrow a pen so he could write down the famous ad-lib.
Split into simpler sentences: Robert Metzler, the show's business manager, believed that the incident had been planned in some way. During the dress rehearsal, Niven had asked Metzler's wife to borrow a pen so he could write down the famous ad-lib.