"Honore Daumier was a master at exploiting the suggestive power of the sketch. He would let his hand move freely, conjuring the figure as he scribbled. His drawings evolved naturally from evocative impulses into more concrete and discernible forms, yet they always retained the sense of energy and movement found in his gestural sketches.

...you can see this progressive development, taken in two different directions. to draw the right-hand figure, Daumier uses a strong singular line that overrides lighter ones and confirms and encloses the body's edges. Daumier recasts and intensifies the original lines of the left figure through repetition to build value that seems to fill out the body from within. A third figure, barely visible in the center of the drawing, suggests how Daumier began, using light marks to coax his figures into being. As the figures evolve, they become increasingly volumetric."

Drawing From Life by Clint Brown and Cheryl McLean. p. 23

Gesture and Sensitive Line:

The use of gesture line allows the artist to capture a subject's movement, form, and character. There is a sense of power, excitement, and life within a form found just below its surface.
A gesture drawing, regardless of subject, portrays that essential form, position in space, and/ or movement of the subject absent of surface detail.
Gesture drawings yield critical underlying information in a nondetailed sense...

A Guide to Drawing by Daniel M. Mendelowitz, David L. Faber, Duane A. Wakeham - pp. 76,77

What is meant by "Sensitive" Line?

Sensitive: having the power of sensation; ready and delicate in response to outside influences; able to register minute changes or differences; degree of responsiveness to stimuli; having power of feeling; of such a nature as to be easily affected.
Sensitive line is sensitive in its description of and response to both inner and outer contours or edges of an object.
Sensitive line is able to register minute changes or differences found along contours or edges.
Sensitive line is responsive to both subtle and not-so-subtle activity found along contours or edges.
Sensitive line has the power to convey a strong sense of volume, mass, form, weight, dimensionality, and space and can also convey a strong sense of feeling.
Sensitive line, in addition to its responsiveness to the information being described or interpreted, it is also sensitive in its own right, independent of subject matter.
Whether it addresses a particular form or exists independently, it can display various qualities including textured or smooth, dark or light, continuous or broken, curvilinear or rectilinear, heavy or delicate, thick or thin, and so on.
But ultimately sensitive contour line can be described as having three main qualities - weight, value, and texture.
Sensitive line is capable of describing a form with simultaneous regard for shadow and light, for position in space (foreground, middle ground, and background), and for perceived physical weight and the effect of gravity on a form.
The shifting quality of weight, value, and texture in line work invites various interpretations regarding light source, spatial position, and weight or grounding of objects.
The quality of line is determined by the artist's response to the medium being used, the surface on which the medium is being applied, and the subject matter with which the artist is concerned.

Achieving Line Variation and Line Sensitivity

There are no specific "formulas" for achieving line quality and sensitivity.
The kind of line employed by the artist is a decision based on the artist's personal response to the form being drawn, and that response is undoubtedly influenced by a multitude of factors.
Lines vary tremendously in character, and each type of line has its own expressive potential.

Drawing Essentials by Deborah Rockman - pp. 52,53