Although every Prophet was intelligent and endowed with a comprehensive understanding and a pure soul, these play no role in God's choice of a Prophet. Most Prophets, including Muhammad, were unlettered and therefore were taught by God. Prophet Muhammad, despite his illiteracy, had knowledge of the past and the future, and insight into every branch of knowledge. He did not attend any school or have any human teachers, yet even his enemies admitted (and still do) that he displayed perfect justice in family affairs, perfect competency in state administration, and perfect command of armies.
Prophets were specially brought up by God. To cite an example, the Last Prophet recalled: "I intended twice in my childhood to attend a wedding ceremony. On both occasions, I was overpowered by sleep half-way [and thus was protected against any sin I would later prohibit] ; and "While repairing the Ka'ba, prior to my Prophethood, I was carrying stones. As everyone did, I wrapped my garment's lower part over my shoulder to avoid injury. Part of my thigh was left uncovered. All of a sudden, the angel I had seen several times in my childhood appeared to me in all his majesty. I fell down and fainted. That was the first and last time I uncovered any part of my body that God ordered to be covered." 
Prophets were protected by God against all sins, for they were created for a special purpose. They were protected from going astray, for even a minor deviation could result in humanity's almost complete deviation.
Prophethood is distinguished by Divine Revelation: And thus have We revealed to you a spirit of Our command. You did not know what the Scripture was, nor what the faith. But We have made it a light whereby We guide whom We will of Our servants. And you, surely you guide unto a straight path (42:52). As a result, Prophets never spoke on their own accord: Nor does he speak of (his own) desire. It is naught but a Revelation revealed (53:3-4).
Prophet Muhammad, particularly when asked about the essentials of belief, would wait for Revelation. Sometimes the polytheists asked him to alter the Qur'an. But as it is a Divine Scripture whose wording and meaning belong completely to God, he would reply, as instructed by God: Say: "It is not for me to alter it of my own accord. I follow nothing, except what is revealed to me" (10:15).
Prophets submitted themselves wholly to God, and fulfilled their mission solely because God commanded them to. They never compromised or deviated from their way in order to achieve success. When confronted with threats or seductive offers, they replied with words similar to those of the Prophet: "If you were even to put the sun in my right hand, and the moon in the left, I will never give up preaching my cause." He knew that the Qur'an is the Word of God, and so bore all hardship and opposition. 
 Bukhari, Hajj, 42; Ibn Kathir, Al-Bidaya, 2:350.
 Ibn Hisham, Sira, 2:285.