Assessment centres – what role should I play?

I am going to attend my first assessment centre and I hoped someone could help me decide on the best way to behave in group exercises.

I am not naturally a leader, I tend to see how the group develops and find my role according to what seems to be the dynamic. However, I am concerned that if I don’t try to lead the exercise or am quiet when the exercise is observed that I will lose out. I feel I have a lot to offer but I don’t push myself forward. How much do I need to get noticed in order to progress?