I enjoy some of those sort of adlibbed interactions with NPCs. Usually I like to provide my players with the choice on whether they want to just get the generic goods for appropriate gold and we can move on, or if they want some sort of special interaction or hard to find item then we go to roleplaying it. This has led to some very entertaining scenes, such as the paladin of Liiria (he fights for your right to party) getting fleeced by the effeminate elvish weaponsmith, the poison merchant who succeeded at poisoning the party assassin as means of presenting his resume, Geryn the rare creature vender who is desperate to sell off his previously-used giant bat mount, and Hans and Frans who were a pair of fairies trapped in a single human body who were trying to free themselves by stealing the soul of the evil party druid.
But I mandate improvised interactions for other scenes. All of my villains, plot NPCs, and other characters are more than willing to talk to the party and I am usually willing to completely rewrite my story based on a well argued dialogue. It's partly why I dislike systems that make it more difficult to create scenes and encounters on the fly. But not all people are comfortable roleplaying like that. It's definitely a skill that some people take to more easily than others.