Which word best describes someone who is not talkative and shy?

Reticent is the most fitting choice to describe someone who is not talkative and shy. The term 'reticent' specifically refers to a person who tends to be reserved and reluctant to communicate verbally, which aligns perfectly with the characteristics of shyness and a lack of talkativeness. This word conveys not just a preference for silence, but also an underlying disposition that restrains one from sharing thoughts and feelings openly.

In contrast, the other words present very different meanings. Assertive describes someone who is confident and willing to express their opinions or needs, which directly opposes the idea of being shy. Verbose indicates someone who uses more words than necessary, suggesting the opposite of the desired characteristic of being quiet. Effusive refers to a person who expresses feelings in a heartfelt and unrestrained manner, again contradicting the notions of shyness and reticence. Thus, reticent stands out as the most appropriate descriptor for a shy individual who tends to be quiet.