I agree with Janice that the primary role of the teacher in using Investigations is to be a facilitator or moderator. There are secondary roles, though. When I introduce a brand new concept, we have we call "yellow paper days," where I lead the whole class in considering a new problem solving approach or concept. Students grapple with the new concept and make notes on yellow paper, which they throw away at the end of the period. (In our class, yellow paper is used strictly for draft versions of work and is never graded.) They love this "risk free" approach to trying something new. When I do this, initially, there is considerable direct instruction and a lot of quiet thinking time as students ponder the new concept. I control the flow of the discussion be making sure students have at least 5 minutes of silent thinking time to ponder the problem (that's a loooong time to be quiet in a classroom!). Students raise their hands when they "get it", but cannot speak until everyone raises their hand or officially declares that they can't "get it." In the early stages of introducing a concept, I am more in the traditional instructor mode. It doesn't last long, fortunately.
Once students are working with a familiar concept, which is really most of the time, my role is to be a facilitator and let them explore their own solutions or strategies. When students are working in pairs or groups, I visit each cluster of students and discuss their thinking with them, answering questions or re-directing individuals as needed. When the class is called back together at the end of the period, my role is to moderate the discussion and let different students come to the board or overhead and share their solutions. Students do the actual presenting and discussion, I just keep order and call students to come up.
In these whole-class summary discussions, I stress diversity of thinking and problem-solving strategies. I have emphasized repeatedly that we all have different perspectives and ways of solving problems, and that we can learn from each other's successes and failures. I love it when a student comes up and shares a mistake and tells how s/he corrected it. I also love it when a student thinks of a solution to a problem that never occurred to me before. I always let students know when they come up with something I have never thought of, as we are all exploring mathematics together. I let them know that I am not the ultimate authority. They can solve the problem however they want, not just "how the teacher does it."