Video prototypes help capture and communicate interaction with paper prototypes in the early stages of design. However, designers sometimes find it tedious to create stop-motion videos for continuous interactions and to re-shoot clips as the design evolves. We introduce Montage, a proof-of-concept implementation of a computer-assisted process for video prototyping. Montage lets designers progressively augment video prototypes with digital sketches, facilitating the creation, reuse and exploration of dynamic interactions. Montage uses chroma keying to decouple the prototyped interface from its context of use, letting designers reuse or change them independently. We describe how Montage enhances video prototyping by combining video with digital animated sketches, encourages the exploration of different contexts of use, and supports prototyping of different interaction styles.