Leads:  

In recent times, the ‘law’ for leading an Ace or a King from a holding of AKxxxx, say, or AKxx is to lead the Ace, if you want Partner to show attitude, or the King if you want partner to show Count. Ace for ‘Attitude’, and King for Count. This works well.  So;  you have ♠AK7654   or    ♠AK54  and you are defending a trump or no trump contract by the opposition, 

1. No Trump contract, you will lead either the Ace or King, or more likely to retain communications, the 6 (4th. highest), from AK7654. If you decide to lead from  AK7654, do you want Attitude or Count ? depends what you find in dummy, but of course it is too late then, so whether you have an outside entry or not, lead the 6 to retain communications with partner. (Exception, if they have shown great strength, might be best to cash Ace and King, keeping it to 11 tricks!) From AK54 against No Trumps, you will normally try to avoid this lead. Why give a trick to the Queen ?
2. Suit contract,  this is where Ace for attitude and King for count is important. With AK7654, you would like to know whether Partner has a doubleton for a ruff, maybe. So you would lead the King. Partner will show Count, i.e.  HiLo if a doubleton, and Lowest from 3 (or of course a singleton);  with AK54, you would like to know Partner’s attitude to your opening lead of the Ace (attitude), i.e.  have they got Q74, say; they will play the 7 (Hi for like, sorry Andrew). If you had say, the 642, then you would play the 2. (low for hate! Sincere apologies to Andy Robson).
   • Lets now go forward to your play when Partner or opposition lead a suit.
3. Lets say, the opposition contract is in a suit, they are playing it. Partner leads a low card in an outside suit. You hold AK74. You must play the King. This identifies that you may hold the Ace. If you carelessly play the Ace on Partner’s (or opposition’s) lead,  you are denying the King.  This is the same at all times. Say Partner or opposition lead a suit, and you are due to play a high card, always play the 9 from J109 - If you play the Jack, you are denying the 10. If you have KQxx  always play the Queen.  Any card in a sequence, then always play the lowest from the sequence, if you are forcing out an honour from Declarer. It is called ‘Communicating with Partner, and helping the defence’. However, note,if you are on lead with a sequence, you always lead top of a sequence. Just something that we have to learn. You lead top of a sequence (especially honour cards,) to show Partner that you have the card below it in sequence. I know, different rules for the similar situation. I never said that bridge is an easy game.
4. Lets say, that Partner (suit) contract, leads an outside suit.  You hold AK in that suit – a doubleton.  Always play the Ace followed by the King, and then switch to another suit; Partners should realise that you have no more cards in that suit. Conventionally with AK doubleton, always play the Ace then the King!
5. McKenny at trick 1.  Partner leads a suit, and dummy has a singleton. You hold the Ace and King (and Partner possibly realises that you have the Ace and the King (bit deep this one, forget asap), if you play the Ace, there is a strong likelihood that you want a higher suit returned – let us say you switch to a trump at trick 2. If you play the King, then maybe you want a lower suit returned. I know, I know, when does this ever come up. Answer, more likely than you think!