Implementing Actors - about message-processing threads?

How the message-processing threads should be implemented in actor system?

1. Should each actor instance have it's own message-processing thread?


2. Should each actor type have it's own message-processing thread?


3. All messages are collected in one place (like central queue) and then served by multiple threads and passed to actor instances for processing?

Would it make sense to have mix of 1 and 3?

Sorry for lame question, i am missing something in this perspective :)