I'm trying to understand how to stretch and scale exactly the keyframes of my animation. Let's say that at the beginning my animation was composed by 66 frames that I want to render at 24 fps. Later I stretched the keyframes to 110 because the character's movements was too robotic. Now I want to come back to 66. I gone to frame 1 on the dope sheet and I scaled down the frames a little,but the position of the characters is not aligned with the background images that I used on the beginning to make the poses. I need to understand what's the method to know exactly how many frames should I add to 66 the make a fluid animation at 24 fps and how can I scale the frames from 110 to 66 to match the starting background images. Check here to see what happened when I tried to scale down the frames. Thanks.
In the animation editor, position the 2D cursor to the frame you want to use as the "origin" for your scale operation (probably the first frame of the animation) and set the pivot point to "2D Cursor".
Select the keyframes you want to scale. Press s to scale, x to scale on the x-axis (time), then type 66, and press Enter. Then press s, x, then type 110/ to scale by 1/110, and press Enter to apply. This will scale your keyframes' timekeys by a factor of 66/110.
The keyframes will probably not align perfectly with entire frames. Blender's interpolation system can handle this just fine. If you want, you can snap them to entire frames by pressing Shift+S and choosing "Nearest Frame".