Finding a Hook

This is an example of a non diatonic functioning diminished chord in another well known standard song, As Time Goes By, starting at its bridge:

Whether the changes are correct or not, you are facing this onstage, sight reading. There is no way the A diminished chord is acting as a diatonic functioning chord, since it is not followed by a half step or the same root. See my workbook.

You must be able to see the chord tone of A dim7. Seeing a chord tone right away is essential to improvisation. Once you see a chord tone, you should see four dominant chords:

The note on the A dim7 chord which does not appear in those four dominant chords becomes tension. Let's examine.

• Gb does not appear in F7 » F7(b9)
  
• B7 is a good match with A dim7 » B7(b9): This has to be the Mixo b9 scale because of the -3rd note of A dim7, which is b9 to B7.
  
• Eb does not appear in D7 » D7 (b9,b13): Tension b13th is added because of the context, that is, proceeded by F-7 and followed by C-7.
  
• Ab7 is also a good match with A dim7 » Ab7(b9): This has to be the Mixo b9 scale because of the root note of A dim7, which is b9 to Ab7.
  

Any one of them will work. You pick one that you like, and blow away.