Question: In "some characteristic C is what distinguishes A from B", does characteristic C belongs to A but not to B, or to B but not to A?

For example:

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> Imposing restrictions on the available resources is what distinguishes computational complexity from computability theory.
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> 
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Answer: 
I think the word you are after is *listener* in everyday terminology.

The speaker speaks to an *audience*. In the audience, you may find some very attentive *listeners*. Everybody in the room will hear the talk, so it might be argued that *hearer* is a possibility.

It depends on the interest of the audience member, though.

Hearing is something you cannot control. Your ears will pick up the sound and send it to your brain. So everyone who hears the sounds will be a *hearer*.

Listening, though, is something that you can control. You can decide to tune out and not pay attention, or you can decide to lap up every sound or word. If you are interested and follow the talk, you will probably describe yourself as a *listener*.

This also manifests in the set term *avid listener*. I'm not sure if such a construct exists for hearing.
The answers given answer the title question of 'hearing equivalent for “speaker”'.

However, the question is actually asked in reference to analysing a **conversation**. In that case, 'speakers' isn't really correct either. Those who speak in a conversation also plays the same listening role as those who do not speak.

In the context of research, the groups would likely be labelled 'speaking participants' and 'non-speaking participants'. In a less formal context, you would talk about 'the people who spoke' and 'the people who didn't speak'.