Why did the manager take the narrator for a detective?

When I go into a bank I get rattled. The clerks rattle me; the wickets rattle me; the sight of the money rattles me; everything rattles me.

The moment I cross the threshold of a bank and attempt to transact business there, I become an irresponsible idiot.

I knew this beforehand, but my salary had been raised to fifty dollars a month and I felt that the bank was the only place for it.

So I shambled in and looked timidly round at the clerks. I had an idea that a person about to open an account must needs consult the manager.

I went up to a wicket marked "Accountant." The accountant was a tall, cool devil. The very sight of him rattled me. My voice was sepulchral.

"Can I see the manager?" I said, and added solemnly, "alone." I don't know why I said "alone."

The manager was a grave, calm man. I held my fifty-six dollars clutched in a crumpled ball in my pocket.

"Are you the manager?" I said. God knows I didn't doubt it.

"Yes," he said.

"Can I see you," I asked, "alone?" I didn't want to say "alone" again, but without it the thing seemed self-evident.

The manager looked at me in some alarm. He felt that I had an awful secret to reveal.

"Come in here," he said, and led the way to a private room. He turned the key in the lock.

"We are safe from interruption here," he said; "sit down."

We both sat down and looked at each other. I found no voice to speak.

"You are one of Pinkerton's men, I presume," he said.

He had gathered from my mysterious manner that I was a detective. I knew what he was thinking, and it made me worse.