Methods for Agile information systems development (ISD) are widely accepted in industry. One key difference in comparison to traditional, plan-driven ISD approaches is that Agile ISD teams rely heavily on direct, informal face-toface communication instead of indirect and formal documents, models, and plans. While the importance of communication in Agile ISD is generally acknowledged, empirical studies investigating this phenomenon are scarce. We empirically open up the "black box" of the Agile ISD process to enhance the knowledge about the communication mechanisms of Agile ISD teams. We conducted a case study at two medium-sized ISD companies. As our primary data collection technique, we carried out semi-structured interviews, which we complemented with observations and, in one case, a survey. Our study's main contribution is a set of so-called social Agile practices that positively impact the direct communication of team members. Our data suggests including the Agile practices co-located office space, daily stand-up meeting, iteration planning meeting, pair programming, sprint retrospective, and sprint review in this set. Furthermore, we investigate the role of more formal, indirect communication in Agile ISD projects. We highlight areas in which formal documents remain important so that a trade-off between indirect and direct communication is necessary.