Class shouldn't define Character

I just want to say that in don't think that just because you play a certain class means you're type casted as that archetype. Choosing a certain class just means that's the way your character chooses to apply their skills when they adventure.

I'm posting this because in the game I run now I had the players pick between 4 pre-made characters that were typically outside of their comfort zones in an effort to get them to be better rollplayers. For two of my players this has worked wonderfully. They are happy playing something new and they said they enjoy their new characters. The other one is trying, just struggles with it. He's the kind of guy that struggles to do the bad thing even in an imaginary setting. So I gave him a Paladin that isn't too far out there for him. His only character flaw is that he's supposed to believe too much in what his order tells him is right, and they can be heavy handed at times. I occasionally have to remind him of his character flaw when he wants to roll insight or work against his order.

The last one is a bit.....okay, an ENORMOUS power gamer. Always gets what he wants, and uses a high charisma to do it. So this time I had him play a rogue. He's a halfling and a noble. His family makes premium locks for clients who don't want things getting stolen. He's adventuring to find the lost trove of treasure hidden by his ancestor long ago. He has a Charisma modifier of 0. Not terrible, but he's not getting any help either. His shtick is that he usually doesn't lie to anyone and is incredibly blunt. He uses his intelligence to get what he wants rather than charm. That's how he's supposed to play, but doesn't. He's always lying and attempting to roll charisma checks, not using what he's good at using at all. This is how most of his characters play, and my plan has completely backfired with him. When I bring this up he just shrugs and says, "Dude, I'm a Rogue! This is what rogue's do!"

I didn't make this looking for help with my problem, I just want to dissuade this kind of thinking. Make a character, not a gimmick.