Roleplaying is a form of "coherent realism", which contradicts the artificial nature of game world. Introducing roleplaying mechanics as part of the game, makes the game more coherent in terms of "story/lore" but less mechanically coherent.
An example: Dwarfs are good with mining, so lets give them +100% mining bonus.
Instead of character earning that mining skill, he gets it by default with no background reason besides that "he is a dwarf, they suppose to be good at it". Ignoring the fact mining isn't inherent property of dwarfs, this absurd perk isn't generated by balance consideration or overall game world(are we suppose to be 100% following some dwarf trope from Tolkien?).

Instead, the roleplaying and gameplay are separated into their own areas of concerns.
NPC interaction and deeply nested dialogue should not be enmeshed with action-oriented gameplay, simply because they interrupt our mode of gaming: Quest-solving mode and Combat-mode require "switching gears" in players mind that should be minimized to reduce immersion loss from mode-switching(that is perceived as speed-bump by player).
(Quests and storyline may also influence player, but its better to restrict it to lore-centric accomplishments not adding quest walls.) .
If a RPG is planned to be quest/roleplay/NPC-interaction heavy, it should diminish its combat elements and reduce their frequency, and vice versa for Action-RPG.
(In effect RPG with diminished/turn-based combat elements, should be rethought as Adventure game with roleplaying and Action-RPG should be imagined as FPS with RPG elements, to avoid losing genre relevance).
"FPS with RPG elements" is the more popular and versatile option of the two(essentially any game from last ten years where you directly control a character can be classified as that).
See also http://videogamedesign.wikidot.com/rpg:combat-centric-gameplay for difference between FPS gameplay and typical RPG.