For instance, the transitive subject of unaccusative (The man broke the vase) and unergative verbs (The jockey raced the horse past the barn) is an agent of causation, whereas the subject of the intransitive variant of unaccusative verbs has a theme role (i.e., undergoes a change of state: The vase broke), and the intransitive subject of unergative verbs has instead an agent role (The horse raced past the barn).
their surface identity notwithstanding, the semantic properties of the syntactic slots of the verbs in each class are very different.