How to force After Effects to interpret footage at a specific framerate?
To answer your edited question:

If your footage is 15fps and After Effects thinks it's 30fps, you need to tell After Effects to re-interpret the footage.
You do this as follows:


Open the Project panel (by default on the left of your screen)
Right click on your footage and go to Interpret Footage -&gt; Main... as shown below.

In the screen that opens up input the desired framerate (15fps in your case) in the textbox next to Conform to framerate. Press OK.



Now After Effects should know that the footage is 15fps.