On 120Hz sets, Real Cinema takes 24p material and displays each frame 5 times to maintain the natural cadence of the film. On a 60Hz set, each frame is displayed twice to keep the 48Hz refresh rate. TruMotion, which you may be confusing Real Cinema with, interpolates the motion between the frames, hence the SOE (soap opera effect). For movies, leave Real Cinema on and TruMotion off. The Real Cinema setting only works when 24p material is presented, so even if the setting is not grayed out (which I think is the case with my LD520 while viewing OTA), it has no effect. However, if you're using your computer as the source, it sounds like there may be some sort of incompatibility between your graphics card and the LG causing them to be slightly out of sync with each other. My BD player outputs at the correct rate without any adjustments and my LG displays 24p when presented.
All I know is that once I play a video using MPC-HC and MadVR kicks in, it sets the desktop refresh rate at 24p and the TV info says that the resolution and refresh rate is indeed at 1080/24p, which turns Real Cinema on. I'm not sure about the TruMotion stuff since I can't seem to find it in the menu.