Animating two characters
"I think my biggest tips would be:
1) film reference with a friend(s). Don't try to do reference of 2 characters acting with each other by yourself by filming one and then the other. You'll miss out on many opportunites to subtly interact that you would discover by filming reference with another person physically in the scene with you. Even if the characters never actually touch, this is a really important step.
2) Be very careful about leading the eye of the audience. It's better to have one character bordering on "dead" than having the audience not knowing where to look. Most importantly, make sure it's very clear which character is talking when. Overacting can wreck a scene, but it can doubly wreck a multiple character scene because you can't tell who is talking or who the animator wants you to be looking at.... Anticipation can help you direct the eye of the audience, as can staging/composition, etc.
Anticipation is one of the big ways though. It's like your secret line of communication with the audience. "hey - look over here because something funny is about to happen!" or maybe subtly moving a character's left hand just before he waves with his right gets the audience to switch over to look at that character and not miss the wave... "