Just wanted to give everyone still reading this a link to the best book on cast studies. 'Charles Bargue and Jean-Leon Gerome: Drawing Course' . Vincent Van Gogh and John Singer Sargent studied form this book. This book is free from the Internet Archive. Or you could pay up to $70 for it on Amazon.
https://archive.org/details/C.BargueDrawingCourse/page/n1/mode/2upCast studies
Decided to have some fun. Every object is nothing more than a collection of smaller shapes. You can divide any complex object into three values of shapes. Light, mid and dark. Look at your hand. Now start picking out the three values as individual shapes. Try to make these shapes as large as possible. These shapes can be broken down into smaller shapes. The more you break these shapes down the more realistic your rendering becomes. But it all starts out with the three value shapes. I have a yearly subscription to Croquis Cafe site on Vimeo. They have a feature called 'fast sketch'. You can set the timer to 15, 30, 45 seconds. One, 2,3,5 or ten minutes for each artist pose. Made all my pose selections and set the timer for 15 seconds. Hit start. As the pictures of models flew by I chose a single body part. Arm, leg, torso, neck. Whatever caught my attention first. Instead of drawing an outline I drew the shapes that made up the area itself. I did this with a pencil. The more i did this the easier it was to find the three values that made up the over all shape. And the better I got at it. By rendering these three value shapes I was actually getting the over all shape. Without starting with an outline. Eventually slowing the timer to one minute to get larger areas of the model. Still keeping the shapes down to three values.
So I know what your asking. "Why are you using a pencil"? Fifteen seconds is just enough time to load a brush and bring it up to the canvas. And it usually takes more than one stroke with a brush to get a completed line. A 'W' with a brush takes four strokes. Compared to one stroke with a pencil. Using a brush would give me more time to think than I wanted . Working fast is a way to force yourself to see the shapes that make up the object. When I go to do the actual cast shadings I'll do that with the brush.