Many of the benefits promised by human-robot interaction require successful continued interaction between a human and a robot; trust is a key component of such interaction. We investigate whether having a person "n the loop" with a robot---i.e., the mere involvement of a person with a robot---affects human-robot trust. We posited that people who press a button on a robot to permit its plan execution would exhibit greater trust than people who merely observe a robot's autonomous execution of the same plan. We assessed trust both toward the robot that participants interacted with and toward robots in potential future use contexts. We found (a) a marginally significant and medium-sized effect of the button press on people's trust in the observed robot (p = .12, d = .52), and (b) a significant and large-sized effect on people's trust in potential future robots, but only in social use contexts (p = .04, d = .68).