looking for ideas that would change expressions in an instant - without being insulting or offensive.
Richard Avedon came to mind shooting the Duke and Duchess of Windsor. They had their "press" faces on. He noticed that they were very attached to their dog. Avedon altered their expressions by (falsely) claiming that he had been delayed when the taxi he was riding in hit a dog en route.
Things like that .....
I found that you need to find a way to relate to them to get beyond their press faces.
I had a similar experience with MC Hammer at a baseball game when I was visiting the US once. He had his press face / fans face on but after a little while of getting to hang out with him and talking to him. Also telling him that my wife was pregnant and we are getting married in a week, his personally changed completely and that is when we were able to get the much better photos with him and have a better conversation.
I would recommend doing some research about them before hand to see if there is something that can cause you to get them more in a natural look.
Part of making a connection with someone and encouraging them to open up, is opening up yourself.
If they have their press face on, it could be because you have your photographer face on.
If you become a person for them to interact with on a personal level then they will interact on a personal level.
It's not about a catchphrase, but sharing a moment with the subject.