A semantic role is the underlying relationship that a participant has with the main verb in a clause .
Semantic role is the actual role a participant plays in some real or imagined situation, apart from the linguistic encoding of those situations.
If, in some real or imagined situation, someone named John purposely hits someone named Bill, then John is the agent and Bill is the patient of the hitting event. Therefore, the semantic role of Bill is the same (patient) in both of the following sentences:
In both of the above sentences, John has the semantic role of agent.