Usually I notice an ensemble member if they are fully in character even when they are just in the background, if they look like they are enjoying themselves is also a plus. Just taking in the part for all that it is and embracing it. Always remember, there are no small parts, only small actors!
I am always cast as a ensemble person in musicals and I have come to totally love it. So much so that I ask the director to give me a small speaking part in the ensemble and nothing more.
I have been asked on several occassions to talk to other ensemble members about what it is I do while on stage. Here is what I always tell them
1. I give my character a name. I am not just "townsperson" but I give myself an idenity, even if no one else knows it.
2. I make up a short back ground story about my character. Why is she there at that moment?
3. If they were to make a play about YOUR character, what would it be about? What is going on in this persons life that no one knows about?
4. Interact with other cast members on stage, but don't over do it.
Have fun!!! Being in the ensemble can be the best time of your life. You don't have to worry about forgetting a bunch of lines, cues or anything like that. You typicaly get a bunch of stage time and a cool costume. Enjoy the experience and don't forget to take photos. I didn't take photos of some of my early plays and now I regret it.