What was the Mariner's effect on the wedding guest?

In Part IV of the poem, the wedding guest is also afraid of the Mariner.  At this part of the poem, the Mariner has just described how Death came to the ship and took all of the crew.  The wedding guest replies: 

'I fear thee, ancient Mariner! 

I fear thy skinny hand! 

And thou art long, and lank, and brown, 

As is the ribbed sea-sand. 

The wedding guest believes that the Mariner is a ghost...

In Part IV of the poem, the wedding guest is also afraid of the Mariner.  At this part of the poem, the Mariner has just described how Death came to the ship and took all of the crew.  The wedding guest replies: 

'I fear thee, ancient Mariner! 

I fear thy skinny hand! 

And thou art long, and lank, and brown, 

As is the ribbed sea-sand. 

The wedding guest believes that the Mariner is a ghost because he is so thin and scary looking.  However, the Mariner reassures the wedding guest that Death spared the Mariner.

The wedding guest is also afraid when the Mariner tells him that all of the dead bodies of the crew rose again and helped him to bring the ship home.  The wedding guest again fears that the Mariner is speaking of ghosts (and might still be a ghost, even though the Mariner assured the guest that he was not).  The Mariner assures the guest, "'Twas not those souls that fled in pain, / Which to their corses came again, / But a troop of spirits blest."  These spirits only rise when it is night and will fall again when the sun is up.  

The Mariner then tells the wedding guest: 

I pass, like night, from land to land; 

I have strange power of speech; 

That moment that his face I see, 

I know the man that must hear me: 

To him my tale I teach. 

There was some aspect of the wedding guest that the Mariner saw that tagged the guest as the next person to hear the Mariner's story.  Thus, the guest leaves when the Mariner finishes his tale with the warning "He prayeth well, who loveth well / Both man and bird and beast."  The reader does not understand why the wedding guest needed this lesson, but it must have struck close to home because the poem states that the guest is "stunned" and is a "sadder and a wiser man."