Applying 3:2 cadence based on Comp frame vs. Source clip frame

When motion picture film was transferred to video, the frame rate was kept at 29.97 for the video, the film chain (the thing that advances the film, was slowed from24 to 23.976, and a 3:2 pulldown was used to interlace the footage. 

When you cut the video, the cadence depended on where the cut was made. Sometimes the frame was A on the first field and B on the second. Other times it was A on the first and A on the second field. Sometimes B, B, sometimes BA. It all depended on when you made the cut.

AA does not mean the first frame is doubled; it means that the first field and the second field come from the first frame of the original footage. Both fields are from an identical time. If you go to another frame, you will get one field from one frame and the other field from the next one. Any motion blur in a frame with fields from different times will be slightly exaggerated, but most people cannot see it unless you pause the footage and examine the motion blur very carefully.

There is no such thing as ABCDD. I don't know where you got that. There's frame A, then Frame B, and you need a combination of fields from the two frames before you get to the next A frame. That is why some frames have both fields from one frame, but every third frame has one field from one frame and the next field from the next frame. That's how interlacing works.

After Effects does not leave holes for the scan lines. You can see the individual fields in interlaced footage if you double the frame rate of the comp. 3:2 Pull Down is only used with interlaced footage. No modern cameras shoot interlaced. Broadcasters still interlace the signal, but almost all content now has identical fields.

I know how this stuff works. I have been in the business since 1970 and shot 16mm and 34mm motion picture films for major networks and some of the biggest advertising agencies in the country for the first 20 years of my career. In about 1975, a friend of mine named Clive Tobin, was a master technician and inventor of the best telecine systems (film to tape transfers). He built a film camera motor that ran at 29.97 fps, would sync to a Nagra recorder, and I bought one so we would not have to add 3:2 pulldown to footage shot for commercials. I got a lot of work because my film to tape tranferes looked better than anyone else's. Pretty soon, a lot of the majors were shooting at 29.97 instead of 24. 

There are very few modern cameras that will shoot interlaced, so the 3:2 pulldown/interlaced question is mute. Dropping 23.976 footage in a 29.97 comp will give you some blended frames, but it would be an extremely rare situation if you needed to interlace the footage to make it look better.