This is from Leigh Anne and we are crazy if we don’t do it every single time an observer comes into the room:
…I make all of my observers participate. It’s my secret weapon….
How to do it? Just start talking about what the observer does. Weave it into the thread of the class. If student A runs to catch the train in a story, ask if the observer runs to catch the train. Play it up. Say it in knowing ways to the class. The observer, not understanding, will have to admit that something is happening in the class that has to do with understanding and acquiring languages.
This is not something a grammar teacher could do. (“Hey class, did Mr. Smith add the correct ending on that verb? Did Mr. Smith or Mickey Mouse add the correct ending on that verb? Class, did Mr. Smith add the correct ending on that verb or on that adjective? etc.”)