The expression practiced privately may be protecting someone from an unpredictable public response.

Rehearsing something in private is usually read as vanity, when it is often closer to caution. Someone who has been laughed at, corrected or misread learns to test an expression where the cost of getting it wrong stays small. The practice is not performance so much as protection. What arrives in public then looks effortless, and the effort simply happened somewhere you were not invited.

When someone seems oddly prepared for an ordinary moment, hold off on deciding why. You can make the public response steadier instead: react plainly, without a joke at their expense. That reliability is what lets the practice eventually become unnecessary.