I think the key point is that most actor models, at least the ones I know, are truly concurrent with preemption, whereas most models based on OO are sequential.

I think the key point is that most actor models, at least the ones I know, are truly concurrent with preemption, whereas most models based on OO are sequential. There is only one thread of execution in the OO program, whereas there is no notion of this in an actor system. All processes execute at the same time, so to speak. There are also some points with the messaging being truly asynchronous, so you can't rely on the order in which messages arrive, usually subject to certain constraints. Erlang, for instance, require that between any two pairs of processes, order is preserved in the messages. Also, creating a new actor creates a new thread of execution inside that actor. This is not the same as creating a new object.