In this paper we focus on the design of Computer Assisted Surgery (CAS) systems and more generally Augmented Reality (AR) systems that assist a user in performing a task on a physical object. Digital information or new actions are defined by the AR system to facilitate or to enrich the natural way the user would interact with the real environment. We focus on the outputs of such systems, so that additional digital information is smoothly integrated with the real environment of the user, by considering an innovative device for displaying guidance information: the mini-screen. We first motivate the choice of the mini-screen based on the ergonomic property of perceptual continuity and then present a design space useful to create interaction techniques based on a mini-screen. Two versions of a Computer ASsisted PERicardial (CASPER) puncture application, as well as a computer assisted renal puncture application, developed in our teams, are used to illustrate the discussion.