1. A computer-implemented method comprising: accessing, from a memory, an executable block diagram, at least a portion of the executable block diagram having message-based execution semantics, the at least a portion of the executable block diagram including a plurality of interconnected message-based blocks, where the message-based blocks send and receive messages during execution of the block diagram, the messages including payloads that remain fixed while the messages travel between respective pairs of the interconnected message-based blocks, and persisting for determined execution time intervals, the determined execution time intervals, during which the messages persist, being defined time periods occurring within a simulation time of the executable block diagram that starts with an execution start time and ends with an execution stop time of the block diagram; adding an observer block to the executable block diagram, the observer block configured to observe one or more of the messages generated by the message-based blocks; executing, by a processor coupled to the memory, the at least a portion of the executable block diagram, the executing producing information associated with the one or more messages being observed by the observer block; and presenting, on a display coupled to the processor, the information associated with the one or more messages being observed by the observer block during the executing of the executable block diagram.