fcp probably won’t do any interpolation, but 25p will work fine in a 50i timeline. if you are editing 25p with 50i material, you may notice that they look slightly different (50i will have smoother motion) but not many people will notice the difference.
however, you can work in a comp that is 25fps, then simply render as 50i by setting the render settings to do a field render (choosing the appropriate field order). that’s a pretty normal workflow too, however renders will take 2x as long.
if you happen to have rendered material or footage in the comp that is not 50i (or 50p) then that material will not get interlaced unless you also enable frame blending (pixel motion) for the footage layers. that will also effect render time….
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW