Chapter 4 of Nathaniel Hawthorne's novel The Scarlet Letter is titled "The Interview". This is because Hester Prynne has just returned from a long session at the scaffold where she is being forced, without results, to confess the name of the man who fathered her child out of wedlock.
We know that Hester has recognized a man in the crowd, and that the effects of this recognition makes her, and curiously enough, her baby daughter, quite uneasy.
This is when the warden of the jail suggests a physician to determine what is wrong with Hester's child sudden outburst of cries. This physician is none other that Hester's husband whom she had thought was dead.
His name is now Roger Chillingworth. He changes his name upon his arrival to the settlement because he does not want to be recognized. The shame of Hester's indiscretion and his thirst for revenge make him want to remain unknown. However, he is a physician, indeed, and he intends to enter the settlement with those credentials so that he can be more involved in everybody's life. He feels that he can probably get to find out who is the perpetrator who commits the act of fornication with Hester.
His name was announced as Roger Chillingworth. The jailer, after ushering him into the room, remained a moment, marvelling at the comparative quiet that followed his entrance; for Hester Prynne had immediately become as still as death, although the child continued to moan.
When Hester recognizes him, the first thing that she fears is that he will prepare some concoction that would kill her child or her.
Wouldst thou avenge thyself on the innocent babe?” whispered she. “Foolish woman!” responded the physician, half coldly, half soothingly. “What should ail me to harm this misbegotten and miserable babe? The medicine is potent for good; and were it my child,—yea, mine own, as well as thine!—I could do no better for it.
We know that Chillingworth does not harm Hester, nor her daughter Pearl. This is not done out of pity, but out of his sick interest of keeping them alive and well enough for Hester to confess. However, his "kindness" comes with a price. For he is determined to find out who cuckolded him and, for this reason, he makes Hester promise to never reveal his identity.
“Thou hast kept the secret of thy paramour. Keep, likewise, mine! There are none in this land that know me. [.....] No matter whether of love or hate; no matter whether of right or wrong! Thou and thine, Hester Prynne, belong to me. My home is where thou art, and where he is. But betray me not!”
Therefore, it is Chillingworth who visits Hester in prison with every intention to cause psychological harm. Although he does help Hester with his knowledge of medicine, he does this for the sole intention of finding out what is really going on. As a result, his payment is to never have his identity reveal. This will cause repercussions as we will find out. For he will use his knowledge to, eventually, identify that the paramour of Hester is none other than Dimmesdale, the respected reverend.