Yes, interpolation to go from 18->24fps, but if that doesn't work out so well for the reasons johnmeyer outlined, there is another way that creates blended frames to fill in the gaps. The
Motion.dll, can do this to, for example, create 6 new frames every second (a new one after every 3) to go from 18->24fps. Each new frame will be a blend of the ones before and after. Ordinarily I don't like creating blended frames on purpose since I spend my life unblending frames but it might do here.
The reason for my question earlier was, if making a DVD is okay, you only have to bring the framerate up to 19.98fps and you can apply 3:3 pulldown afterwards to output the required 29.97fps. That way you'd have to create only a new frame after every 9, rather than after every 3. Only 10% of the frames would be interpolated. I've never worked with Blu-Ray and have no idea if soft pulldown is allowed.