It's not an act, she's well known in animal circles as a huge supporter of animal causes. Amongst other things she's been vegetarian since she was about 12, is an Ambassador for Battersea and ran the London Marathon for the Born Free Foundation. She's Patron of Born Free and is more than just a figurehead, being actively involved in projects such as travelling to India with them to rescue tigers. She also does a lot of things like this:
https://youtu.be/gJnF1mJ2yj0
Amanda has a big fan base and her support brings in a lot of money to these charities and animal welfare organisations.