My only contribution to this topic would be to tread carefully with regards to peoples' comfort zones.
I game with a 6-member group that possesses 3 multi-decade gamers (myself included) who can RP til the cows come home. We use accents and gesticulations, and are prone to standing up to monologue like thespians.
Another member is new to table-topping, but has already warmed up to the RP thing and is digging in like a free buffet.
The last two are the focus of my attention here. One is a complete newbie who has social comfort issues, and the other is a new-to-tabletop nerd-grrl with similar social phobias. (sidebar: gotta love geeks/nerds/gamers, ultimate paradox of introverts and social outcasts who feel driven to pursue fundamentally social hobbies!)
These two people have a great time in most aspects of the experience, but are simply not comfortable doing voices or dialog. They stretch as much as they can, but simply can't do it, and feel uncomfortable. They contribute by understanding the situation/goals, and coming up with their characters' actions in a simple descriptive manner, ie. "I distract the guards by utilizing my natural feminine attributes. Once they are fully distracted, the rest of you can sneak by and open the side door for me when I make my way over there." No "RP" per se, but still a non-violent solution using skills and a clear character concept.
So in our D&D campaign, I had to accept that some players would do a full-on enactment of events including dialog and sometimes props, and some would simply summarize intention, goals and strategy. Neither method is wrong, and each way leads to the same conclusion.
You can't reward one style of play and punish another. Giving bonus XP/rewards to someone who naturally "acts" and offering nothing to another person who contributes in other equally valid ways only serves to reinforce bad self-esteem in social situations. This also lessens the fun factor for some players, who then feel they have to push beyond their own comfort zones so that they can get a "treat" of some kind for being the player you want them to be, and not the player they want to be.
The trick is getting those two styles of play to co-exist comfortably at the same table. Reward ideas and contribution, not the method of execution. As long as everyone contributes in some form and everyone is smiling and involved, the GM is doing their job. Don't let the RPers run table in every social encounter, and always equally praise the contributions of the whole table, provided they deserve it.
Overly simplified Example:
GM to RPer: What do you do?
RPer: Standing up from the table, I boldly proclaim that we are brave adventurers travelling all over this great land seeking treasure and the cause of freedom for all... (blah blah blah as the party tries to negotiate a fee for routing a cave full of orcs that are plaguing a village)
This goes on for a minute or two, with some dialog, until we wrap it up with an opposed skill check.
GM to non-RPer: Ok, so what are you doing while all of this is happening?
Non-RPer: I want to know if the magistrate is buying it.
GM: OK, gimme a check. *dice rolls*. Well, for the most part, you think he's sold, but you get the feeling he needs one last push.
Non-Rper: Well, then I say some stuff that appeals to his sense of family. About protecting them. Stuff about what lengths he would go to, to keep them safe. That kind of thing.
GM: OK, gimme the check you want to use for that, and let's see what happens.......
End of overly simplified example.
Both methods are used here, and both methods work just fine. However, I don't give a reward to the RPer and not the other, since both have contributed equally to the situation.
A reward for the successful negotiation could be that the magistrate offers a fee that is substantially higher than what the party was initially going to charge.
To summarize, just be aware that some people just aren't comfortable behaving a certain way, and they shouldn't be ignored or punished for it. The goal is having fun, and a good GM should be conscious of the many ways in which people go about it.