Finally I got it! This is
The People v. Clark Kent by ML Thompson, a favourite of mine!
Shame on me 
I didn't remember it before! Clark is imprisoned because he killed someone... but has he really done it? it's a must-read story
Back in a few with a new quote...
Ok, this is easy:
"I'm not building you a baseball
field," I blurted, thinking we might as well get that out on the
table.
"Huh?"
"You know, like in that Kevin Costner movie, where the baseball
players come out of the cornfield. Field of Dreams. We're not
doing that here. This isn't even my corn."
He smiled. "I'm not a baseball player... and you don't look a
thing like Kevin Costner."
Well, that was gratifying, even if it was completely beside the
point. "Are you... you know... real?"
His eyebrows shot up to his hairline. "I certainly think so."
I went down the steps and edged a little closer to him. It was
dusk now, and it was getting harder to make out details. "I
mean, real in the sense of being... well, alive."
"You think I'm a *ghost*?"
"I kind of *hope* you're a ghost," I told him.
Simona
