Ask why a figure looks stiff and the usual answers are about anatomy. The real cause is almost always earlier: the drawing began with a construction and never had a gesture underneath it. The first line of a figure decides its energy, and everything drawn afterwards inherits that decision, including the parts that are anatomically flawless.
This is the most transferable single idea in figure drawing, and it costs nothing to adopt. It is also the one that separates drawings that look alive from drawings that look assembled, at every level of finish and every degree of stylisation.
What a line of action is
It is a single sweeping curve that describes what the whole figure is doing. Not the spine, though it often follows it, and not an outline. It is the direction of the intention, and it can extend beyond the body, running from a foot through the torso and out along an arm into empty space.
One line, drawn fast, before anything else. If it cannot be drawn in one stroke the pose is probably not clear yet, and clarifying it now is much cheaper than discovering the ambiguity after an hour of construction.
Gesture carries intention
Viewers read what a figure is doing before they read what it looks like, and that reading comes almost entirely from the gesture. A figure with a strong line of action communicates instantly even when heavily simplified, which is why a five-line sketch can be more legible than a fully rendered drawing built on nothing.
This is the same priority described in the piece on hands, where gesture precedes anatomy for the same reason. Intention first, structure second, detail last, and the order matters more than the quality of any individual stage.
Exaggerate, then pull back
Most gestures are drawn too timidly. Pushing the curve further than the reference shows, and then constructing within that, produces a figure with more life than a faithful copy. Animators call this pushing the pose and it applies equally to still work.
The safeguard is to exaggerate the gesture and keep the anatomy honest. A pushed line of action with correct construction reads as energetic; a pushed line with pushed anatomy reads as broken. Knowing which to distort is the difference between stylisation and error.
Gesture in two-figure work
Two figures each need their own line of action, and the relationship between the two curves is what makes a pair read as connected or as adjacent. Curves that answer each other read as interaction; parallel curves read as two people doing the same thing near each other.
This is the compositional half of the contact question in the piece on drawing two bodies in contact. Roughing both gestures together, before either figure is constructed, is what produces arrangements that look mutual rather than assembled.
Practising gesture
Thirty-second poses are the standard exercise and they work because they remove every option except the gesture. There is no time to be careful, which is exactly the point. A page of twenty thirty-second drawings teaches more about energy than one careful hour does.
Aim for one clear statement per drawing rather than for accuracy. Most people improve fastest when they accept that these drawings will be wrong and judge them on whether the intention reads, which is the practice discipline described in the piece on what improves figure drawing.
Keeping gesture through to the finish
Gesture is easy to lose during rendering. Careful construction, clean-up and shading all tend to average a pose toward the neutral, and a drawing that was alive in the sketch arrives inert. Keeping the original gesture visible on a layer and checking against it periodically is the simplest guard against that.
The final check is to squint at the finished drawing and see whether the curve is still legible. If it has disappeared, the finish has flattened the drawing, and a small amount of re-emphasis in silhouette or line weight will usually bring it back.
Beginning a figure with what it is doing
One fast curve, pushed a little further than seems necessary, before any construction. Keep it visible, build honest anatomy inside it, and check at the end that it survived. That is the whole practice, and it changes drawings more than any amount of anatomical study.
Characters here are fictional and unmistakably adult, as set out in the reader agreement.