Was Fat Ass, the character from Shawshank Redemption, actually innocent?

We don't know. His character doesn't appear in the source novella and the two scenes in the film are all the background we've got on him.

If it makes you feel any better, around 2.5-5% of all US prisoners are actually innocent of the crimes for which they were jailed so there's at least a 1:20 chance that he was wrongly imprisoned.

Dialog from the movie (and I've seen it like 7 times, but reading the dialogue, it's really quite brilliant, every line)

http://www.imsdb.com/scripts/Shawshank-Redemption,-The.html

FAT-ASS

    OH GOD! I DON'T BELONG HERE! I 
    WANNA GO HOME! 

28 INT -- HEYWOOD'S CELL -- NIGHT (1947) 28

            HEYWOOD 
    AND IT'S FAT-ASS BY A NOSE.' 

29 INT -- CELLBLOCK -- NIGHT (1947) 29

The place goes nuts. Fat-Ass throws himself screaming against
the bars. The entire block starts CHANTING: 

            VOICES 
    Fresh fish...fresh fish...fresh 
    fish...fresh fish... 

            FAT-ASS 
    I WANNA GO HOME! I WANT MY MOTHER.' 

            VOICE (O.S.) 
    I had your mother! She wasn't that 
    great! 

The lights bump on. GUARDS pour in, led by Hadley himself.

            HADLEY 
    What the Christ is this happy shit? 

            VOICE (O.S.) 
    He took the Lord's name in vain! 
    I'm tellin' the warden! 

            HADLEY 
        (to the unseen wit) 
    You'll be tellin' him with my baton 
    up your ass! 

Hadley arrives at Fat-Ass' cell, bellowing through the bars:

            HADLEY 
    What's your malfunction you fat 
    fuckin' barrel of monkey-spunk? 

            FAT-ASS 
    PLEASE! THIS AIN'T RIGHT! I AIN'T 
    SUPPOSED TO BE HERE! NOT ME! 

So, if we take his words at face value

"I don't belong here, I want to go home" "I want to go home I want my mother" "Please, this ain't right. I ain't supposed to be here"

He never says "I'm innocent", just "I'm not supposed to be here", an innocent person would probably say "I'm innocent".

What it suggests to me is that he was a dupe. He might have drove a friend to a crime scene, perhaps not knowing his friends plan, or, perhaps not wanting anything for himself, or perhaps by coercion. Any of those would fit with what little we know of his character.

Based on the dialogue, I wouldn't call him "Innocent", but arguably charged harshly for an offense he didn't have a direct or major role in. Certainly not a repeat criminal.

Well, we don't know for sure and we never find out. The only thing I can tell you is this: Some people arrive at prison in a state of shock or denial. Usually not a hardened criminal, but even they can be subject to a breakdown if they're going into a pen that contains a rival. I've seen people cry themselves to sleep, trying desperately to reason with the fact that they're going to waste many years of their lives rotting in a cell. And I've heard people say, "I ain't supposed to be here", simply because they never thought they'd get caught committing a crime.

The one thing a repeat offender knows is that you do not show emotion. You don't smile, and you don't cry. If you cry, you're gonna become someone's prag in no time.

Given my experience, I'd say at a minimum that Fat Ass was a first-timer. Whether or not he ever committed a crime, or thought a plea bargain was going to get him into a country club and instead landed him at Shawshank, is purely speculation.

I presumed that either (a) he plea bargained to get into an easier prison, but (for whatever reason) was sent to do hard time in the state prison, or (b) his crime was white collar and his lawyer said that he wouldn't do hard time.