I've been tinkering with IExternalizable, but I've noticed some unexpected behavior. I've got this class:
public function readExternal(input:IDataInput):void {
    input.readObject();
    input.readObject();
    input.readObject();
}
public function writeExternal(output:IDataOutput):void {
    output.writeObject("first string");
    output.writeObject(424242);
    output.writeObject("second string");
}
But when I try to serialize this class using AMF and send it to a remote server (via RemoteObject), Charles shows me that the request looks like this:

But it seems wrong that my serialized object is leaking out into the rest of the request.
So, what am I doing wrong? Is there some part of the documentation I've missed?