I'm not asking how to make every character be played as I would play them. I'm asking how to handle a situation in which the character that the player presents to me as a GM (with complete back-story, concept, etc., included) ends up being played vastly differently than I think that character would actually act given their initially presented concept. How can I look at a player and say, 'I don't think the character you presented to me would do that,' in a way that doesn't make it seem like I'm accusing them of poor role playing, or trying to play their character for them?
For example, one of my players decided he wanted to run a half-orc Paladin. The character he came up with had been born in a paladin order, was the son of its leader, and was held to higher standards than everyone else because of his parentage, and race. He described the character as 'being the type to follow the rules as-written, no matter what.' (In his own words, he wanted to play a 'lawful stupid' paladin.'
Well, as a sort of safety-net, I gave him a modified magic item. It was a phylactery of faithfulness, with the advantage that he did not have to have his character consider an action before being told if it was or wasn't evil. However, if he tried to go forward with an evil action, it would give him a small jolt (in the form of 1d4 nonlethal) to make him realize that what he's doing is wrong. Basically, if he asked me if he could do something (ex: throwing a chair at the performing NPC bard, to 'get his attention') I would say no, and he would take the d4. (Looking back, I see that this was a bad idea, and bad GM tactic.)
He took exception to this, as, in his words, "I wasn't letting him play his character as he thought it should be played," and, "even though he asked if he could do those things, that didn't mean his character was thinking about them." It came to a head in one game when, after telling him that 'grappling and throwing the rogue at the group of Orcs earned him a d4' he completely stopped playing. Literally, stopped. He refused to even speak, and under the circumstances, basically ruined the game for everyone.
I do not condone what he did, nor am I asking how to deal with that specific situation. I'm asking how I handle a player whose idea of what his character should do in play is nearly opposite to what I (the GM) think it should be given their background?