Around the theatre groups where we work, my brother Michael and I are often confused for each other; and even more frequently, someone will know which name each of us bears, but will simply ask for "an Angelico" without caring who responds. Why? Are we fungible? To an extent, but not completely. To be more precise, people duck-type us.
Duck typing is, broadly speaking, the notion that you don't ask what something IS, you instead ask whether it's able to do what you want it to do. Popular in the Python programming community, it's the idea that you don't care if this is a file as long as you can write to it and read from it; you don't care if this is an integer as long as you can add to it and subtract from it; you don't care if this is a list as long as you can step across its elements.
When someone needs a volunteer, they don't care whether his name is Fred or Joe. They don't care whether he's cast, crew, or random passer-by. All they care is that he be able to do what's needed, willing to do it, and not presently occupied with another task. That's duck typing. It is, in fact, the ultimate in fairness and equity; there's no racism, no sexism, no ism of any kind. If you will work, you will be accepted.
Of course, there are times when other criteria are added (such as 'Weapon Proficiency: Screw-gun'), which will still work the same way. But often, tasks simply require a willing pair of hands - everyone has the necessary hardware and skills, and yet valuable volunteers are treasured like Hildebrand's wise remarks. There always seem to be two or three times as many jobs as people to do them; if you want the respect of pretty much everyone involved in a show, just offer to help!