*Warning, this is something of a rant*
In my opinion gameplay shouldn't be complimented by roleplaying. It should be the other way around. Game mechanics should be designed around the narrative/roleplaying in the game.
One of my favorite experiences as a GM was actually running a relatively short-lived D&D 4th edition game with a few of my friends. There was a tale about an “emotional plane” (home-brewed stuff of course) colliding with our. One of the fights was against a “Insanity Elemental”. He had 3 little “invokers” which could activate an “insanity” effect on the players. I knew the players pretty well so I tossed out the rules for “insanity” from the PHB and went for something more abstract. If he successfully effected them with this spell-like ability they would become “insane”. An insane character got a save at the end of their turn (10+ ends) but one of their actions was controlled by the GM. (Movement, standard, free, ect) By describing the various things that MADE THEM INSANE I didn't even have to take the action for them by the end of the fight! For instance, the spell caster had a lava-lamp like thing apparently smothering him so the only obvious answer to his character was to magic-missile himself in the face to get it off. (his choice- not mine) Another one actually took off his armor and got naked due to the apparent seething flames belching forth from his armor. It was an example of rules almost coming about as a result of good roleplaying.
In that same campaign the players where effected by a “Lust Elemental” who compelled every member of the party to seek out “the dagger”. The players didn't know WHAT “the dagger” was but (as our mantra became that session) they “really wanted the dagger”. EVERY question about it ended with the line “...but/and you really want the dagger.”. Even when the players where doing something completely unrelated I would bug them for a moment and say something like:
“Do YOU want the dagger?”
“Um.. I don't know? Do I?”
“Yes... you want the dagger”
“Do YOU want the dagger?”
“That's right... you want the dagger”
This went on for the ENTIRE session, to the point where there was no need to use any in-game mechanics to FORCE them to roleplay a magical compulsion towards the dagger- they simply wanted the dagger. In the end it was actually another lust elemental who had turned herself into a dagger. I didn't even bother to make rules for the magical compulsion- the players just really wanted the dagger. It was an interesting experience, the roleplaying guided the group to almost make rules for themselves. I guess if you paint a vivid enough picture or make something EXCESSIVLY reprehensible then you can guide a game simply by roleplay rather than by clunky rules. Do I think this would work for a group of inexperienced roleplayers? No. The people I had with me where all excellent and where doing this out of sheer enjoyment- but that's how pen and paper games should be right? Just sitting around laughing you ass off while playing a compelling game. To this day when I talk to the players they still something ask “By the way, do I want the dagger?” and all I can do is smile and say “Yes! You REALLY want the dagger.”