Thanks for your interest. While it is true that we are looking for students involved in a variety of different activities -- there is by no means any strict requirements for this. The program is looking for students passionate about science and excited about learning through the model that Integrated Science has. So long as you can talk about some life experiences (whether just 1 that has really affected you and motivated you, or a variety that have each shaped who you are), that would be sufficient for the supplemental application.
And we have a mixture of shy and outgoing students within the program, I would make no real distinction there. The program does emphasize skills in scientific communication -- whether that's written, visual, or oral, and groupwork is also a huge component, where you'll need to effectively interact and work with other students -- so only in this regard, may a more outgoing student have an advantage, but these are skills that you will learn to develop through the program if not.