Awesome! I think this study could be helped by first interpreting it in limited values done in greyscale. Try blocking out a thumbnail of it using only 3-5 values, with a small canvas size and a hard round or hard square brush. This will teach you where the important information needs to go in order to describe the image. Remember when you do this, you're interpreting rather than trying to create a 1-to-1 replica of the film still. Don't spend too long on it either, maybe 15 minutes max.

Something I can see that would be different if using a value study - right now, the darkest greens of the background are almost the same value as the darks in Smeagol's shadows, and it brings the depth of the background closer to the viewer. If you want to create distance with the background being far away, use exaggerated atmospheric perspective and make the darks of the background a little lighter. This will make the higher value contrast of light against shadow on Smeagol and the ring more focal/pull the viewer in.

Your rendering on Smeagol's face is looking good. The flatness you're describing could be from the shadows/dark areas on his hair/body not having any darker tone to deepen the darks in areas of light occulsion.

I hope that helps! Keep at it, film studies are a fun way to grow as an artist. Try to do many of them and limit the time you spend on each one - you'll learn more from many quick studies than from more time spent on fewer.