When receiving a remote event from a client to server, the first parameter on server is the player that fired it, So for Example
in a local script
Local RemoteEvent = -- to the remote event
RemoteEvent:FireServer() -- add some parameters to your likings
in a server
Local RemoteEvent = -- to the remote event
RemoteEvent.OnServerEvent:Connect(function(Player)
--do the fast cast here
End)
I believe this is the solution you wanted?