Model sheets solve a real problem: a character who changes shape across a long project. They also create one, because an artist working directly from a sheet tends to reproduce its poses and its energy, and a character drawn that way is consistent and inert.
The two outcomes come from different working orders. Drawing from the sheet produces stamps; drawing freely and checking against the sheet produces consistency without stiffness, and the difference in effort between the two is nil.
What a sheet should define
Proportions, structure, the construction of the face, the wardrobe, and the character's default posture. These are constants. What a sheet should not be doing is providing poses to copy, since poses belong to the scene and the moment rather than to the character.
Numbers are what make a sheet checkable, as argued in the piece on what a character sheet must answer. Head height, shoulder width in heads, where the waist sits. Those can be verified in a finished drawing; a vague resemblance cannot.
Gesture first, then check
The working order that avoids stiffness is to draw the pose freely, from gesture, without reference to the sheet, and only then compare proportions and correct. That keeps the energy of the initial drawing, which is what the line-of-action argument in the piece on gesture is about.
Reversed, the sheet supplies the pose and the drawing inherits its stillness. This is the single most common cause of characters who are technically on model and visually dead, and it is entirely a question of sequence.
Expression is not on the sheet
A sheet can show an expression range, and it cannot show the expression a particular moment needs. Artists who work only from the range produce characters who cycle through five faces, which readers notice quickly and describe as the character having no interior life.
Better to treat the sheet's expressions as construction references, showing how this face is built when it does various things, and then to draw whatever the scene needs using those principles. The mouth structure in the piece on drawing the mouth is the underlying mechanism.
Sheets should be updated
Characters develop over a long project and the drawing improves. A sheet made in the first month becomes a constraint that holds the work back, and artists frequently keep obeying it long after they have outgrown it.
Redrawing the sheet once or twice during a long project is normal practice. The important part is doing it deliberately and applying the update consistently, rather than letting the character drift and then rationalising it afterwards.
Drift versus development
The distinction is whether the change was chosen. Development is a decision applied everywhere from a known point; drift is an accumulation nobody noticed, and drift is what makes chapter one and chapter eight look like different books.
Comparing current work against the sheet at intervals is what catches it, which is the same periodic check recommended for a whole cast in the piece on auditing a cast.
Sheets when more than one person draws
With a second artist involved the sheet stops being optional and its quality determines how much correction work there is. Numbers, orthographic views, construction notes and the awkward angles are what a collaborator actually needs, and they are usually the parts that are missing.
It also needs to state what may vary. A collaborator who does not know which properties are fixed will either freeze, copying the sheet exactly, or improvise and produce inconsistency.
What a sheet cannot do
A sheet records how a character is built and says nothing about how she behaves, and behaviour is what readers actually recognise. Two artists working from the same sheet can produce characters who feel like different people, because one has understood how she moves and the other has only matched her measurements.
That argues for a written note alongside the drawings: how she stands when she is waiting, what she does with her hands, whether she takes up space. A paragraph of that is worth more to a collaborator than another turnaround view, and almost no sheet includes one.
Staying on model without going flat
Put proportions and construction on the sheet, draw from gesture first and check afterwards, treat expression references as construction rather than as a menu, update the sheet deliberately once or twice, and compare current work against it periodically.
Characters here are fictional and unmistakably adult, and adult proportion is one of the fixed properties a sheet records.