Large screen displays are being increasingly deployed in public areas for advertising, entertainment, and information display. Recently we have witnessed increasing interest in supporting interaction with such displays using personal mobile devices. To enable the rapid development of public large screen interactive applications, we have designed and developed the MAGIC Broker. The MAGIC Broker provides a set of abstractions and a simple RESTful web services protocol to easily program interactive public large screen display applications with a focus on mobile device interactions. We have carried out a preliminary evaluation of the MAGIC Broker via the development of a number of prototypes and believe our toolkit is a valid first step in developing a generic support infrastructure to empower developers of interactive large screen display applications.