Hei! Great drawings!
I can see that you are implanting the box logic to your figures, and volume lines that give volume to shapes. The thing about quick drawings is exactly what you are mentioning, its good exercise to stop worrying about results. So critiques have to reflect that :) I cant pick on every little mistakes.
What I find hard working with boxes, is that I restrict myself too much to the edges of the boxes, and my drawings become too tied up. It's like: the lines restrict, at the same time- Box thinking and volume lines are essential for describing form and to get away from 2 d contour-drawing. When your goal is to draw from imagination, from any angle- Boxes are the thing. So, think both box, and think outside the box :D Practice boxes, practice action lines without boxes, and implement them. Keep it loose. You are doing great!
Your face drawings could improve from the same idea. Facial features are more or less in line with eachother, exept- They have some expressions- things move a bit around. On the front of the face: The eyes, tip of nose, and chin, will be more or less aligned, and drawings often benefit from this. A way to implement box logic would be to make small marks on natural points, eyebrows, nose tip, and chin- with parallel lines. Instead of drawing a box, you just suggest it. The ear will be on the other side of the visible "box" and will have another direction, still aligned with the eye. If the face is at a more normal distance- the lines are closer to paralel- and you would get away with it. If the face you are drawing is supposed to be very close- these suggested lines will taper and be steeper- towards a vanishing point.
Boxes don't need to be square, and the box logic doesn't even have to show any box at all. Just directions.
Hope it makes sense :)
