I think it could be different if the intention is that the message (or method body) is always executed in the context of the called object. E.g. in an own threa

I think it could be different if the intention is that the message (or method body) is always executed in the context of the called object. E.g. in an own thread, which is used instead of the thread/callstack of the the caller, and which is also exclusivly used even if the method is called from multiple threads - which would elide the need for synchronization. However as far as I understand most implementations that talk about messaging (e.g. ObjectiveC) do exactly the same thing as plain method calls. The difference seems to be that there is more dynamic in the "messaging" (can send any message to an object or can write a "method" that processes arbitrary messages) - but for me that sounds more like the difference between static and dynamic languages than as a completely different messaging concept instead of methods. I don't know Squeal and Smalltalk so maybe I'm missing something here