How is that even possible? Surely there must be a type tag associated with each object if you can dispatch on it, just like in other languages.
A variable of an interface type is a pair consisting of a type descriptor and a concrete value. It is only when a concrete value of a statically known type is passed to something expecting an interface type that such a type tagged value is constructed.
Actually the distinction between heap and stack is not determined by how it is referenced. The compiler is free to stack-allocate any value as long as it does not escape the function. We can do this because pointers are opaque; there's no arithmetic.
The main point is that Go does not have classes. You just define methods on values. Values are no bigger than the data they represent, so you don't suffer from the same kinds of overheads seen in other "OOP"-centric languages.