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Peter seeing him, saith to Jesus,.... Peter took a great deal of notice of John, and very likely understood, that he meant by his rising up and following Christ, to signify his readiness for service and suffering in the cause of Christ: and therefore says,

Lord, and what shall this man do? The phrase in the original is very short and concise, "Lord, and this what?" The Arabic version renders it, "and this, of what mind is he?" it looks as if he was of the same mind with me to follow thee; but it is better rendered by us, "what shall this man do?" in what work and service shall he be employed, who seems as willing as I am to serve thee? or it may be rendered thus, "and what shall this man suffer?" shall he suffer at all? and if he shall, what kind of death shall he undergo? what will become of him? what will be his end? how will it fare with him? this he said, partly out of curiosity, and partly out of concern for him, they two being associates and intimates, who had a strong affection for each other.

What shall this man do? - This question probably means, “What death shall he die?” But it is impossible to ascertain certainly why Peter asked this question. John was a favorite disciple, and perhaps Peter suspected that he would have a happier lot, and not be put to death in this manner. Peter was grieved at the question of Jesus; he was probably deeply affected with the account of his own approaching sufferings; and, with perhaps a mixture of grief and envy, he asked what would be his lot. But it is possible, also, that it was from kindness to John - a deep solicitude about him, and a wish that he might not die in the same manner as one who had denied his Lord. Whatever the motive was, it was a curiosity which the Lord Jesus did not choose to gratify.