Complex sentence: Rex Harrison was cast in the role of Caesar, much to Hordern's dislike; the animosity between the two was still obvious, although they agreed to endure each other's company for the sake of the film.
Split into simpler sentences: Rex Harrison was cast in the role of Caesar and much to Hordern's dislike. The animosity between the two actors was still obvious, although they agreed to endure each other's company for the sake of the film.