One that is holding a conversation [duplicate]

One who is good at making conversation is a conversationalist.

The standard term is interlocutor, but if the word conversation (or an equivalent) is already in the context, people often prefer the less "wordy" term participant.

As @Simchona says, there is also conversationalist, but ordinarily this specifically means someone who is habitually good at keeping up the flow of conversation, so that other participants feel interested/relaxed/motivated and will thus also actively participate. It doesn't normally mean someone who talks a lot it such a way that others listen without saying much themselves, nor does it mean someone who happens to be in a conversation right now.

A neutral word would be converser, though it's not too common.