Students are often shy of contributing or asking questions in large groups. They fear that their contributions will be wrong, or trivial, and that they might get a rebuff. There are some solutions to this:
If you want the lecture to be interactive always start early, in the first few minutes of the class so that the students do not go into a passive mode. It is much harder to get students to participate, if you talk for twenty minutes before opening it up to them.
Always start your questioning with simple questions that students will find reassuringly easy to speak out on. Once their confidence has been built up you can move on to more complex issues.
Always thank a student and give a positive response to a contribution or question.
If a student has the answer wrong, never ridicule or dismiss them but lead them to a more appropriate answer by careful questioning.
Get students to formulate their questions or answers in pairs or threes. They will then feel more confident in sharing their ideas with the larger group.
Ask students to formulate questions and then hand them in at the end of the class (or during a break). You can then address these at the start of the next session and this also provides you with feedback on any areas of misunderstanding or uncertainty.