Complex sentence: Cartoonist Sergio Aragonés, who was filming nearby, dressed for his role as an armed policeman, abruptly encountered Feldman while introducing himself, frightening Feldman and possibly induced his heart attack.
Split into simpler sentences: Cartoonist Sergio Aragonés was filming nearby, and dressed for his role as an armed policeman. Aragonés abruptly encountered Feldman while introducing himself, frightening Feldman, and possibly inducing his heart attack.