In augmented reality, people can feel the illusion of virtual humans (VHs) integrated into a real (physical) space. However, affordances of the real world and virtual contents might conflict, for example, when the VHs and real objects "collide" by occupying the same space. This implausible conflict can cause a break in presence in real-virtual human interactions. In this paper, we address an effort to avoid this conflict by maintaining the VH's spatial and behavioral coherence with respect to the physical objects or events (e.g., natural occlusions and appropriate help-requesting behaviors to avoid implausible physical-virtual collisions). We present a human subject experiment examining the effects of the physical-virtual coherence on human perceptions, such as social/copresence and behaviors with the VH. The basic ideas, experimental design, and results supporting the benefit of the VH's spatial and behavioral coherence are presented and discussed.