Posing: Action and Reaction

 The most obvious simplest way to compose 2 characters talking to each other is

have the one who is talking lean towards the one who is listening
the listening character leans back at less of an angle than the speaker leans forward

Aim the eyes at each other too for good communication between the characters

Of course you don't always want to use that formula. It wouldn't be natural.

Here the character talking is leaning back and thinking out loud while the other character leans in the other direction-but she connects to him with her eyes.

 In continuity, you should vary the degrees of action and reactions and have the characters take turns.

Variety of poses and expressions is more natural than repeating the same poses or having each character in the same pose in a scene.

Oh and thanks to Ger Apeldoorn, Mark Christianson, Mark Kausler and Yowp for making these beautifully drawn comics available to the world!