I don't know if there is a best way to plan a scene, but I would like to mention here some effective things that I have picked up in my career as an animator that might help someone reading this blog.
1. Know how your scene (shot) fits into the whole of the story. If you haven't seen a whole cut of the film yet, at least ask about the context of the shots surrounding yours.
2. Find out what the director wants (hopefully through a direct 'launch' from the director themselves, or from your supervisor)
3. Look at the story boards and study the poses. Those poses and composition were great at telling the story, so make sure you study them and find out why they worked in the sequence as a whole. Then, expand upon those ideas with animation to really bring the characters to life.
4. If no story boards were made for your shot, create some thumbnail story poses of your own. These 'story telling' poses may become your keys later on.
5. Listen to your dialog track and write it out both regularly and phonetically. Listen for subtleties and nuances. Listen for cadence and accents. Listen for breaths and pregnant pauses. Make note of all of these things.
6. Act out your shot. Explore as many ways as you can to tell the story simply and in an entertaining way. Ask a friend to act it out; they may have different ideas or mannerisms that you might not have thought of yourself.
7. Video record yourself acting out the shot
8. Make thumbnails from your extreme (key) poses.
9. See how you might make those thumbnail poses stronger; better silhouette, stronger line of action.
10. Start blocking out the key poses and have fun! :)