A.T. Robertson Commentary Luke 5:12

"And it came to pass, while he was in one of the cities, behold, a man full of leprosy: and when he saw Jesus, he fell on his face, and besought him, saying, Lord, if thou wilt, thou canst make me clean." — Luke 5:12 (ASV)

Behold (κα ιδου). Quite a Hebraistic idiom, this use of κα after εγενετο (almost like οτ) with ιδου (interjection) and no verb.

Full of leprosy (πληρης λεπρας). Mr 1:40 and Mt 8:2 have simply "a leper" which see. Evidently a bad case full of sores and far advanced as Luke the physician notes. The law (Leviticus 13:12f.) curiously treated advanced cases as less unclean than the earlier stages.

Fell on his face (πεσων επ προσωπον). Second aorist active participle of πιπτω, common verb. Mr 1:40 has "kneeling" (γονυπετων) and Mt 8:40 "worshipped" (προσεκυνε). All three attitudes were possible one after the other. All three Synoptics quote the identical language of the leper and the identical answer of Jesus. His condition of the third class turned on the "will" (θεληις) of Jesus who at once asserts his will (θηλω) and cleanses him. All three likewise mention the touch (ηψατο, verse 13) of Christ's hand on the unclean leper and the instantaneous cure.