How does Stevenson show Dr. Lanyon’s shock when he discovers the truth in extract 4? (PEEL)

From the text:

“O God!” I screamed, and “O God!” again and again; for there before my eyes—pale and shaken, and half-fainting, and groping before him with his hands, like a man restored from death—there stood Henry Jekyll!

My life is shaken to its roots; sleep has left me; the deadliest terror sits by me at all hours of the day and night; I feel that my days are numbered, and that I must die; and yet I shall die incredulous. As for the moral turpitude that man unveiled to me, even with tears of penitence, I cannot, even in memory, dwell on it without a start of horror.