Synonym for 'expression of character through posture and/or countenance'

Though it doesn't usually include countenance, deportment may do the trick.

Lexico:

British The way a person stands and walks, particularly as an element of etiquette.

‘His whole aspect and deportment is such that it suggests that he can't even sit still and read a book in a quiet and un-cheeky manner.’

A few examples:

A Justice of the Peace, by his deportment, (that is, his demeanour, dress, conduct, and general behaviour—in both his public and private life), will not only command the respect and admiration of his colleagues and the public, but will aslo exemplify by such deportment the honour and dignity of the office. (Legal Supplement Part C to the “Trinidad and Tobago Gazette’’, Vol. 40,No. 184, 28th September, 2001)

He had early entered into military life; had borne both a Dutch and a French commission; had seen real service, had travelled, was master of the English language, and evinced, by his deportment, that he was no stranger to the society of gentlemen. ( The Sceptical Young Officer, Rev. Dr. J. M. Mason, of New York)

Ancus Marcius called Tarquin into his presence and was allegedly so affected by his deportment that he soon made it his habit to consult Tarquin on a number of topics, both public and private. (Tanaquil (fl. late 7th–early 6th BCE), William S. Greenwalt , Professor of Classical History, Santa Clara University, Santa Clara, California)

I rather like mien.

air (see AIR entry 1 sense 3c) or bearing especially as expressive of attitude or personality : DEMEANOR.

of aristocratic mien

of somber mien

the mien of a choirboy

(Source: Merriam-Webster)

If that doesn't quite fit the bill, see the "Choose the right synonym for mien" section, op. cit..

BEARING, DEPORTMENT, DEMEANOR, MIEN, MANNER, CARRIAGE mean the outward manifestation of personality or attitude. BEARING is the most general of these words but now usually implies characteristic posture. a woman of regal bearing DEPORTMENT suggests actions or behavior as formed by breeding or training. your deportment was atrocious DEMEANOR suggests one's attitude toward others as expressed in outward behavior. the haughty demeanor of the headwaiter MIEN is a literary term referring both to bearing and demeanor. a mien of supreme self-satisfaction MANNER implies characteristic or customary way of moving and gesturing and addressing others. the imperious manner of a man used to giving orders CARRIAGE applies chiefly to habitual posture in standing or walking. the kind of carriage learned at boarding school

Bearing has something to do with posture. And perhaps demeanor works. It is mentioned above (and in comments) but seems to be more toward outward behavior. Mien may be slightly preferable in that it is "a literary term referring both to bearing and demeanor. "

Is this what you're looking for?

mannerism

a characteristic and often unconscious mode or peculiarity of action, bearing, or treatment

The actor can mimic the President's mannerisms perfectly.

[Merriam-Webster]

Someone's mannerisms are the gestures or ways of speaking which are very characteristic of them, and which they often use.

His mannerisms are more those of a preoccupied math professor.

[Collins]