"Most players believe that being Dummy is rather boring and try to avoid the role as the main duty is to place played cards in the direction of NS or EW depending on who won the trick. However the number of rights and constraints thrust upon you may surprise you. Here are just some of them.

As Dummy you can:

1. Warn any of the players that a quitted card is in the wrong direction, but only if you spot the lapse before the next trick is started.

2. Attempt to prevent Declarer leading from the wrong hand by saying something like, ‘I think you are in Dummy’. However you must not point out the error once a card has been played.

3. Try to prevent a Defender from leading out of turn. Even so you must remain silent once the card has been played.

4. Attempt to warn Declarer of a possible revoke by for example asking, ‘No more spades, partner?’

But what about the limitations placed on Dummy?  Once again here are just a few of them.  During the play of the hand, as Dummy you are not allowed to:

5. Comment on any irregularity committed by any player.

6. Study an opponent’s system card.

7. Say how many tricks have been won or lost.

8. Call the TD on your own initiative.

9. Hover over a card or suit in dummy in anticipation of declarer’s next play.

At the end of the play of the hand as Dummy you may:

10. Draw attention to a perceived irregularity.

Condition 6 is often broken at club level. The danger here is that you may unintentionally be giving unauthorised information (UI) to your partner.

Condition 9 is quite important.  At one time or another, I suspect most of us have committed this offence in an attempt to save time. However it is prohibited under the laws. The TD can award an adjusted score if he/she believes that Dummy was indicating a possible line of play to Declarer.

Technically a procedural penalty against your side is a possibility for disregarding any of the rules, but in club competition the TD would probably just point out your error and take no further action. 

Note: The list above is not exhaustive; full details can be found in Sections 42-43 of the 2017 laws."