Improv is a beautiful mishmash of individual and group expression… all brought together in a collective of 4 or 5 minute moments (longer, if you are talking about long-form). Either it is 2 individuals or 3, 4, 5 or 6… it doesn’t matter because the process is always the same… listen, agree, emotion, character.
Even when you do this you still have to find your place in a scene… and once you figure that out, you have to make it important to you. Sometimes your place in a scene is not in the scene at all but on the side supporting the scene with your readiness, sometimes your place in the scene is being the main focus of the scene, sometimes it is being a supporting character (although I hesitate using that term in the context of improv because in actuality we are all supporting characters) and sometimes your roll is to be a lamp post or a rug. Whatever your place in a scene, it is important… so important that you must make that aspect of the scene the most important to you.
There is a tendency to just wait in your character for your next opportunity to speak. The key is to enjoy the process of improv, each and every character and moment.