AG asked: I would love to hear from some of the experts at the club the answer to this question (both as Declarer and the Defenders): ‘What’s the first thing you do when the dummy is laid down?’ I know the short answer is ‘Make a Plan’ however, I’d like to know more than that.  What are they looking for, what makes their hearts glow? What makes their hearts sink? What are their strategies for play?

DF responded: 
  1. I try to review the bidding and the opening lead even before dummy goes down. Did it seem straightforward or was there difficulty or uncertainty in the bidding? If it was competitive (especially with unequal vulnerability), does it seem as if there was a sacrifice bid? As declarer or the opening leader's partner, what does the opening lead suggest in the context of the bidding? As defenders, how many points (and, dividing by 3, how many "useful" cards) is your partner likely to have?
  2. When dummy goes down, as declarer I try to consider the hand suit by suit. In a NT contract I initially count my top winners and then the winners I can develop and compare them with my contracted target. If I have a danger suit I check how many tricks I need to lose to establish the additional winners I need to make and let this influence which strategies I can afford to try. Sometimes this also is influenced by which of the opponents is the danger hand and whether one or both of the opponents seem, at that point, to be unaware of the danger suit. All of these factors go into making a plan, as does considering whether my order of play might be restricted by a shortage of entries.
  3. As declarer in a suit contract I tend first to count losers suit by suit, and then consider the options to avoid any of these losers. Some of these options will need me to hold off drawing trumps to wait until losers can be ruffed in the hand, or due to trumps providing the only entry to that hand.
  4.  As declarer I also try to consider dummy and our combined hands in the context of the bidding and the opening lead. Are our honour cards well or poorly placed? Are we likely to be in the same contract as is played at other tables, in which case can we make an overtrick? Or if we are in what is likely to be a good contract that others might not bid, I try to play it as safely as possible to cope with unlucky suit breaks. 
  5. As a defender, I compare dummy with the hand I imagined from the bidding, and also whether I have useful honour cards over dummy's or declarer's apparent holdings. I try to assess whether this is a hand where declarer may struggle to make their needed tricks and the defense should play safe and wait for our tricks, or whether declarer or dummy has a good side-suit that will bring the hand home? If the latter, should I  take risks to try to win the setting tricks before the side-suit can be established? 

AD added: DF's reply is very comprehensive. One thing I would add is this:

When in a trump contract, first priority is normally to draw trumps. So when dummy goes down are there good reasons why trumps should not be drawn as soon as declarer/dummy are on lead? Examples where trumps should not be drawn include:
  1. Dummy has a shortage and declarer has losers in that suit. Look for ruffs in dummy which may be impossible if trumps are drawn.
  2. Both declarer and dummy have shortages in different side suits. A cross ruff may be required, which may be difficult if trumps are drawn immediately.