You're new to that side of the project; you know that and so does the supervisor, so I wouldnt worry too much about your supervisors' expectations. If you feel you need more time grasping the basics of a subject before getting creative and developing your own ideas, just explain that, and they'll very likely understand. I know of one student who is doing an interdisciplinary PhD in maths and genetics, yet knew next to nothing about genetics when he started. Discussing those less familiar topics with your fellow students could also help you pick up the new concepts more quickly.