I have been working freelance with interactive and immersive media for several years. I usually come in somewhere between creative technologist, developer and prototyper: understanding what an interaction is supposed to feel like, choosing a practical technical route, building it, testing it with people and making it survive outside a development machine.
The work can be very software-heavy — Unity, creative coding, networking, sensing — or involve physical interfaces, projection, small electronics and on-site production. I am comfortable entering projects early, when the interaction is still vague, as well as later when an existing prototype needs to become reliable enough for an exhibition or event.