The model-of-the-moment says her close friend stills gets butterflies in her stomach before taking to the runway and has a secret vulnerable side underneath her confident, carefree persona.
Joan told Fashionista.com: ''I think [Cara's] very comfortable in who she is as a person, so she really puts out whatever she wants people to know. She's very free-spirited, and that's one thing I love about her. Something that people don't know? She gets nervous sometimes doing shows.
''We all have a little butterfly. I'm like, 'Really? After so many shows?' And she's like, 'Yeah, I still get kinda nervous.' I think that's cute. It shows a little vulnerability, regardless if the world sees you in a certain way -- we're still human and we still have different emotions, as well.''
The 25-year-old star - who is the face of Estee Lauder - currently ranks on the top spot on Models.com and says she has worked hard to make a name for herself.
Joan said: ''I've always aimed to be the best and be very competitive within myself. Being named the no. 1 model was basically a sign saying that you've worked hard, and people who have worked with you are acclaiming your good work. I secretly wanted it, for sure.
''I even told Lara [Stone] one time -- we were in London shooting -- and I said, 'I'm coming for your no. 1 spot.' She laughed and I said, 'Oh you're an icon already, so you can give it up.' ''
Perry appeared on James Corden's 'The Late Late Show' on Monday and addressed her long-standing rivalry with Swift.