To make your question short, and to see if I understood it correctly, we're talking about a player who made his character a certain one and roleplays it entirely different. You added that you think that it comes from inexperience, and that he created this character after you said "no" to some "freak-character"-ideas. You want to help him roleplay the character he created.
As I see it, this problem is made from two smaller ones. The first is that he doesn't see his character as interesting because the character "is normal and normal is boring". The second is that you wanna help him understand why the way he plays the character does not fit the story-world of your game.
Helping him understand that "normal is not boring
This is the more important problem, as it stands in the basis of the entire problem. If he'll see that normal characters can be interesting his "anti-persona" will perish and he'll roleplay a normal character and not a freak one. The main trick here is to show him that normal characters are not entirely normal, i.e. "no person is like the others". In order for that to work, we need to give the character depth.
The easiest way to give depth to a character is through internal conflicts. Having goals and all is nice, but without something that blocks oneself from achieving them it is far less interesting. First thing to do is to go over his character's background and see if he implemented there an internal conflict for his character. If so, show it to him and talk with him about it. If no, sit with him and help him to come with one. The internal conflict doesn't have to be extravagant, but it needs to be there. An example one might be that he loves Vincent's sister but secretly hates Vincent himself, or another like Loves the sister but thinks that he's not good enough for there. I'll take the second one as an example for this section.
The conflict gives us a few things, a few added benefits. It gives the character 2 conflicting goals: "Get the sister and prove that I'm worthy". Now, with those two we also get a kind of an achieving-plan: "If I'll show her that I'm worthy, by getting something amazing done, she'll want me and I'll be able to get her". More than that, the character gets the knowledge that each advancement in order to achieve one goal will drive the other one to the far end.
But the first conflict is even more interesting. The character here has the knowledge that he needs the brother in order to save his lover, but he just can't stand being near the brother. He'll drive the mission onward for two reasons but he'll have doubts about his lover- if he'll marry her he'll be stuck with this brother of hers.
To make long story short, simple conflicts can show the player that even normal characters are interesting and unique. When combined with goals they force the character to take certain steps along the roads, to commit certain actions along the way, that he won't want to do but will make him doubt himself and question himself and see that his problem are far more interesting than those of every freak that he'll encounter.
Another nice way to help him see the importance of conflict is through showing him and analyzing with him certain protagonists that are normal people, from the stories and movies and series (of any form)that he likes. He'll see quite quickly that the conflicts make them interesting.
But he may say that it is not enough. For that there are a few more literary tools that might help him see why normal people are interesting. The first one is having flaws (internal or external) and the second one is using "The Ghost".
Flawed characters are characters that just like normal people aren't perfect. Those flaws can be internal (self-doubts, for example, or a mild paranoia) or they can be external (they're look frightens ordinary people, for once, or a missing hand for the other). The idea is that the character has to deal with the flaw, and one day to find the strength to overcome it. The fight for the overcoming act makes the character far more interesting. A nice example of that can be seen in The Rain Man, where he learns at the end that he can count on strangers/"dumb" persons like he's brother. Another nice example can be seen in the story of The Ugly Duckling who although looking terrible learned to acknowledge himself and to accept the way he looks, to accept his difference.
"The Ghost" is an event from the past that just like a ghost haunts the character to this day. Again, trying to cope with it is what builds a deep character. One example for this can be seen in the movie Inception, where we literally have a ghost- Cob's wife. Another example for this can be seen in the movie Casablanca, where he has to deal with his broken relationship with Ilsa. This Ghost is far more interesting as the originator of the Ghost actually comes back to his life. In Frozen we see another kind of a Ghost- the act that one feels guilty about. Elsa actually killed her sister.
All of these techniques are there for one reason- to make regular people interesting, to give depth to the characters, to make them human beings with goals and drives and psychology.
Helping him see that his character doesn't fit the world
After he understands that he doesn't have to be a freak in order to be interesting, he will be far more understandable about playing a character that fits the world. Then, try to explain to him as calmly as you can what it is in the way he played his character that doesn’t fit the world.
Explain to him that the characters are in a world where being a freak is bad, where achieving one's goals is the ideal. Each and every one for himself, as the saying goes. Give him examples from the way he played his character and analyze with him, in a one-on-one conversation where his way of acting came from. Use the background he created to illustrate to him where your problem comes from.
Then ask him what problems he has with his character, and together try to find a solution. Maybe let him be just a little bit freakish. Maybe he needs to just create a different character. This is basically between you and him. After that show the updated character to the group and get their approval.
When combining those two, you'll get a player who his far more willing to both play the character while also seeing the problems with the way he played his character before.
Combining the two solutions
When combining the two solutions you get a better player, who understands for the future also how to create regular characters that are not freaks yet far more interesting than those freaks will ever be able to be. Furthermore, you get a player who is willing to play his character as written while still making the character fit into the world. Hope any of these helped you.
Well, this kind of psychological change is inevitably going to show up in the character's behaviour to some extent, unless he's a really gifted actor. So I think you need to tell the GM, so that he can play NPCs' reactions to this. If you can't trust the GM not to tip your hand, then you are trying something too subtle and/or complicated for the group, and need a different group.
It seems legitimate to let the other players realise what's going on from your character's behaviour, but be warned that they may realise this much earlier or later than you intend. This is a fact of life in role-playing: it's a shared creation, and you can't decide what other players will do or think.
It's very possible that if the players realise what's going on before your character has become utterly bloodthirsty, they will try to save you from this fate. They are entitled to try to do that. They have, after all, taken you on as a companion whom they trust in dangerous situations. You wouldn't complain about them saving your character's life down a dungeon; they can reasonably try to save him from the effects of PTSD.
Besides, an arc where you suffer and are redeemed seems more interesting than one where you become a dedicated killer and are likely executed for it in the end.
Best Answer
What your players are doing probably is role-playing. They just aren't speaking in character.
Some people don't like to speak in character, being afraid of looking silly or just plain feeling uncomfortable to do so. Especially if you have one person that is very verbose and eloquent in character that can make more introverted people shy away from speaking in character.
And I think that's not a problem.
Role-playing is descriptive anyway, so I think it's perfectly fine to just describe what the character is asking. Preferably, they should describe this a bit more, so you as a DM have something to work with.
In your example of asking the orc whether he stole the map, the orc would obviously say no - unless the characters make him say the truth by intimidating him, convincing him they're on his side etc.
Now this can be done in-character, or by describing: Either: "I approach the orc. >>I know you have the map. Give it to me, or else I will turn you into a cockroach!<< I support my threat with a display of magic, using the Cantrip spell" Or: "I approach the orc and threaten him with a small display of magic using the Cantrip spell. If he doesn't give me the map, I will turn him into a cockroach!"
Now it's up to you to ask for an intimidate roll in both cases. And both cases are actual role-playing. One might even argue that they are at the same level of immersion, but that's a highly subjective topic.
A very different thing is, when your players don't describe HOW their characters try to achieve something. This is the crucial role-playing bit! This is something you really have to get out of the players - and you can just ask them for it: "I ask the orc if he stole the map." "Well, if you just ask him, he's just gonna say no. You need to try something - why should he tell you he commited the crime?" "Uhh, well, I intimidate him." "Okay. Want to add a bit more detail?" ...
The Angry DM's article on interactions has given a lot of insight on this very issue. You might like to read it yourself.
Seeing how a lot of people want to "reward good roleplaying". I think this is a fair point, but again, don't confuse "roleplaying" with speaking in character. If you reward speaking in character, look at the other side of the deal: A shy player might feel at a disadvantage, just because he is uncomfortable to act. On the other hand, in my orc example above I included the "display of magic" in both cases. This could give a small bonus to the intimidation check, because the character has done something to back up his threat. Now this "backing up" can be done both in-character and in the descriptive approach.