There is a smaller group of sanctuaries, though, that offer an internship program that allows for a more 'full-immersion' experience. These internships are almost always unpaid, and may require a refundable deposit to participate, however they can provide a great learning and work experience to anyone interested in animal care or behavior. During these internships, the sanctuary typically offers humble living quarters (if any at all), and usually require a minimum commitment of 4 to 8 weeks, with the option of staying longer (i.e. 12 weeks or more). It is also a requirement that you live as a vegan (or at least vegetarian in some cases) while on the property, and to not bring animal products into the sanctuary out of respect to the residents.
Some sanctuaries may have certain requirements to intern (e.g. be currently enrolled in college), while others may be open to anyone age 18 and older. Many sanctuaries that have internships will also give college credit to those that want it. And because they offer credits, the summer months are highly sought after and tend to fill up quickly as that is when students are out of school. So depending on who you are and why you want to intern, applying for the autumn, winter, and spring months may give you a better chance at being selected.
Below is an in-progress list of sanctuaries that currently allow internships.