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When the design closely aligns with the real world problem it solves, communication pathways are natural and you don't really have to care much about them. What matters is the Actor's role and making sure it represent a strong domain concept. The rest follows naturally.

But to be fair, it's never that simple and you always end up with some part of a system that's less "well-designed". In that case,figuring out who talks to who can quickly become a nightmare.

Actors are great on the paper, but to benefit from them, you need great understanding of your domain. I tend to use it later in the development process, on specific part where the domain is rich and understood.