[to3m]

If the TV displays N fields per second, and the C64 produces N fields per second, then you'll see one at a time. The only effect of the interlace is to shift alternate frames down by 0.5 scanlines. TV phosphor decay is too rapid for anything else to happen. By the time the next field starts the previous one is already gone. (I took some short-exposure shots with my camera and it looks like the phosphor decays from white to black in less than 1/6th of a frame.)

(Well... it's been a long time since I've seen a C64 plugged into a TV, but I don't see how anything else could happen! You definitely don't get an alternating effect from a BBC Micro.)

[dragonwriter]

You are accounting for phosphor decay but not persistence of vision.

[vidarh]

If you use an Amiga, which has interlaced graphics modes, you see very easily that persistence of vision is too short to make those images seem stable unless the colours on each pair of lines are very close to each other. It works quite well for photos etc., but is awful for e.g. text or graphics with sharp lines at high contrast.

[to3m]

Try it and see!