According to "some apparently authentic accounts," W. Montgomery Watt's Muhammad, Prophet and Statesman describes him (at page 229). He was:
...of average height or a little above the average. His chest and shoulders were broad, and altogether he was of sturdy build. His arms were long, and his hands and feet rough. His forehead was large and prominent, and he had a hooked nose and large black eyes with a touch of brown. The hair of his head was long and thick, straight or slightly curled. His beard was also thick, and he had a thin line of fine hair on his neck and chest. His cheeks were spare, his mouth large, and he had a pleasant smile. In complexion he was fair. He always walked as if he was rushing downhill, and others had difficulty in keeping up with him.
What was his demeanor like? Watt continues:
He was given to sadness, and there were long periods of silence when he was deep in thought; yet he never rested but was always busy with something. He never spoke unnecessarily. What he said was always to the point and sufficient to make his meaning clear, but there was no padding. From the first to the last he spoke rapidly. Over his feelings he had a firm control. When he was annoyed he would turn aside; when he was pleased, he lowered his eyes. His time was carefully apportioned according to the various demands on him. In his dealings with people he was above all tactful. He could be severe at times, though in the main he was not rough but gentle. His laugh was mostly a smile.
This, according to historical accounts, was the 40-year-old man who went into the Cave of Hira during the Arabic month of Ramadan in the year 610 C.E. Muslims believe on the 17th day of that month, Ubu'l Kassim was first visited by the angel Gabriel who, over a period of about 22 years, dictated in Arabic the words one finds today in the Koran.
It started when, alone in the cave, he heard someone say the Arabic word "Iqraa!"