Practice improvising with multiple partners but one partner per scene. Create scenes that deal with conflict. For example, your neighbor is complaining about your loud music. This exercise will teach you how to work with the energy of numerous characters and teach you how to stretch the boundaries of your abilities.
Look at your fellow actor in the eye closest to the camera. This will reveal your eyes more clearly to your audience and you will be able to convey more emotion. Most successful actors have remarkable eyes and this is because the eyes communicate emotion more than any other physical feature.
Don't blink if you are trying to communicate authority or power. Blink if your character has lower status than your partner's character -- make the blinking natural by not forcing it.
Create a history for your character. While you read the lines, "I just want you to be happy," by creating a history between your character and your partner's character; you will have a more-genuine emotion from which to deliver your lines.
Think of an event from your past from which you can generate emotion. For example, if your character is sad, think of a situation in your past when you were sad. Bottle that emotion inside of you and then release it when the director calls "Action."
Be subtle. Less is more in acting. Keep the majority of your emotions inside. An effective actor is usually calm on the outside -- unless the director is asking for a lot of physical action -- but inside, their mind is racing. This is communicated through your eyes, tone and subtle movements of your facial muscles.
Prepare your lines until you are sick of them. You don't want to think about your lines while acting. You should have them memorized to a point where their delivery is habitual. The emotion and tone, however should seem like you have read the lines 100 times and this is where the improvisation in Step 1 and playing off your fellow actor's energy comes in play.