Hi there Guelio! First and foremost, I appreciate how you capture the emotions and expressions of the faces in your 30 second/1 min sketches. That helps the viewer relate to the mood of the subject even with very few lines. In your 1min, 5min and 10 min sketches, you do a really. good job at placing the features and maintaining the proportions.
To improve drawing faces in different angles I would practice drawing skulls and contour images in varying angles in order to get a feel for how the proportions and placement shifts as the head turns. You have a pretty good understanding to begin with so it would probably be a good idea to practice with creating midtones, shading, and highlights to create more lighting in your drawings.
Your renderings of the mouth would benefit greatly from studying anatomy, specifically an Écorché of the face. Keep in mind, a sketch and a finished piece are two different things. If you wanted to make a really convincing pencil drawing use a high resolution reference image and spend a few days on it to get that realistic look you're striving for. I personally prefer high contrast references because the dramatic lighting allows me to create more depth and form.
In my study, I've found that creating finished pieces massively improved my sketching techniques. Practice making finished renderings of skulls and references and your sketches will start to look like finished pieces themselves.
Greatest works on your loosest and most totally dynamic facial forms. Please keep up the greatest works!
Still, I'm not getting enough of the fastness and the most loosest and lightest lines in your poses of faces. How would you kindly love to do 1 hour of 30 second expression attitudes, using your underhand position?(120 facial sketches, with a 5 minute break towards the end of the session) As a result, before your start some of the most realistically complex forms of the faces, mind if we suggest doing the strongest strokes of your facial lines of movement, so that way, you'd be able to go with the flow in terms of your facial storytelling expressions. For even really most details, then I'd suggest this video down below.👇
This one is about how you would and should start out with the loosest abstract pass with anything, especially faces.