Antonyms for "play joke on" include serious, sincere, earnest, and honest. These words imply a lack of humor or playfulness and suggest a more grounded or serious approach to a situation. Instead of making fun of someone, one could be honest and respectful, or sincere and straightforward. Furthermore, instead of playing a practical joke on someone, one could be earnest and forthright, communicating clearly and directly. By employing these antonyms, individuals can communicate with greater clarity and respect, avoiding negative or hurtful interactions.