The assignment
Iris treats a dinner party like a three-course menu: something small with drinks, something more involved between courses, and something for the whole table once the plates are gone. Never more than one trick per course, reads her note. The food is the main event. You are the bread basket. She is a fictional character and has never cleared a plate in her life.
Read the room
Ask the host before the evening, not at the table. Some hosts would love a card trick over coffee; some have planned a toast for exactly that moment.
Each trick needs a clean patch of table. The 21-card in particular needs room for three columns of seven cards. Wait until the plates are gone, and never deal next to anything with gravy in it.
The Kruskal count works best with one volunteer counting and everyone else watching. With several people each following their own secret chain, the odds that you name every final card go down. One volunteer, one honest prediction.
If something goes wrong
The 21-card fails only if the deal is off, so if it misses, count the packet; it will have twenty or twenty-two cards. The Kruskal count misses about one time in six even when everything is done right. Say so, reshuffle, and try once more. A table that has just watched you admit a miss is more impressed by the hit that follows.