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Before you join the line
Check TKTS Live and make a shortlist of shows you would actually see. TDF advertises same-day performances and next-day matinees at discounts of up to 50%. The exact mix changes throughout the day. Check Times Square hours for your date, especially for holidays, and confirm which performance you are asking for at the window.
At the window
Tell the agent how many seats you need and whether you can split your party or take a different section. Ask for the total and seat locations. If a preferred show disappears from the board, choose your backup or check later, as productions can release more tickets. The live list is the best source for day-of inventory and the booth page gives the current policies.