When you create a layer in a composition, and that layer is based on a video footage item, the composition checks at each composition frame to see (sample) the data from the footage item. If there isn't a single frame of data from the footage item that exactly lines up with the frame in the composition, then the composition samples from the footage frames on either side and does one of several things: It can either just pick one of the two frames, or it can just mix them together, or it can do something much smarter and try to reconstruct the image that would have existed between the two frames.What it does depends on what frame blending setting you're using.
When you're using a footage item that has a higher frame rate than your composition, and both frame rates are relatively high, you don't need to worry about this at all, as the default settings tend to work just fine.
To sum up, changing the frame rate of the composition just changes how often the composition samples the image data from the footage item. It does nothing to the footage item itslef and doesn't affect how fast the video from that footage item plays in a layer.
To interpret a footage item as being at another frame rate, or to conform a footage item to a frame rate, you have to use the Interpret Footage dialog box.
For more information, see "Frame rate".
thanks a lot