You can apply footage interpretation options to multiple footage items at once, or you can change the interpretation rules to always interpret a certain kind of footage in a certain way. See this page for instructions.
It seems this issue is more complicated that I initially thought.
Ok, when AE initially loads my AVCHD clip (MTS file), it says this:
(*) Use frame rate from file: (29.970 fps)
Separate Fields: Upper Field First
If I leave that alone, and try to Roto Brush the footage, I get a warning...
Frame rate mismatch. For best Roto Brush results, set the composition to 59.94 fps to match the layer source.
Ok, so to fix this I interpret footage and change to:
Separate Fields: Off
The Roto Brush warning goes away... but... now my video is interlaced!
The footage is 1920x1080 30p (29.97fps) 24 mbps AVCHD video produced by a Canon Vixia HF11 solid state camcorder.
I also have some complaints about the quality of the AVCHD rendering produced by AE, but will start a new topic for that.
Page 124 of the Canon HF11 camera:
Recordings made with the [PF24], [PF30] frame rate are converted and recorded on the memory as 60i.
Canon's PF30 setting actually *is* interlaced. The camera "films" 30fps and 24fps, but then stores them to memory as interlaced frames. After Effects and Premiere are doing the right thing here by interpreting this footage as Upper Field First.
After Effects QA