Acting styles had to change. In silents, exaggerated movements and gestures helped convey what was happening, above and beyond and dialogue put up on the screen. With sound, more natural movements became the norm, as actors could now use voice inflection and tone to indicate their emotional state, as well as speak. Actors now had to memorize scripts, often several pages/minutes worth in one scene. This meant that simply being good looking was not enough to make one a great actor or actress. Despite common stereotypes, actors with brains now became a hotter commodity.
it also meant that some actors and actresses couldn't make the transition from silents to sound. Either their voices did not sound good, or they couldn't break the habits learned in silents, including the exaggerated gesturing.
It also widened the types of movies that could be made. Romantic movies and comedies, as well as action films were fairly easy to do in silent format, but more complex stories, such as mysteries or dramas, were very difficult to do. With the ability to have actors convey information to the audience with monologues or dialogues, writers could develop far more convoluted story plots. This also led to movies tending to be longer, as story lines and characters could be more fully developed.