When sending messages, the sender only encodes the name of message and the receiver decided at runtime which method is to be activated. When calling method, the caller encodes at compile time the address (or at least offset) of the target method, to which it directly jumps at runtime. However, from the programmer’s view the observed behavior is (typically) the same, both cases are synchronous and thus block until the “called” method returns. I mention this because other languages (or networking systems) have true message sending where control immediately returns to the “caller” and the called method is run in parallel, ie asynchronously. So Smalltalk is kind of a hybrid in that sense. (And if you delve into the land of JIT optimization, things are put upside down anyway, and we are back at calling or even inlining methods… :)

Sending a message is not calling a method as I have come to understand it. The reason it is called sending a message is because the receiver of the message gets to decide what to do, so it is not the direct invokation of a method. Without that indirection then yes it would be equivalent to a C function call. It doesn’t need to be asynchronous to be considered a message send.