The ~30 frames is made up from 24 frames so running time doesn’t change. Every frame is split into two fields (odd and even) and 2 out of every 5 frames is made up so that the fields are from different frames. This is not a very good way of presenting the movies (theoretically 40% of the movie looks like shit on newer displays ).
[size=7pt]Better way is to have the movie on the disc at 24fps progressive. Each frame only contains information from that frame and the movie can be played like that on all progressive displays (computer monitors, newer televisions) and if it is played on an interlaced display (older tvs) the player can convert the image into 30fps on the fly.
For example almost all of Wild Easts discs are presented in this interlaced 30fps manner (right now I can remember only one progressive disc, Death Rides A Horse) but… their discs are very often converted from a 25fps PAL source. I think (but maybe I’m wrong) it would be a very simple operation to just slow the whole film down by 1fps to 24fps and then release it progressive, but WE seems to (because the running times remain the same as the PAL source has) convert them into 30fps directly from the 25fps. So they have the crap frames from interlacing and as an extra the problem PAL has (audio pitch). All of the old SPO releases from Japan are also bad PAL to NTSC conversions.[/size]