One of the challenges most speakers would face is to engage the audience or get them involved. To have a non responsive crowd would not be a good thing. The speaker with all his enthusiasm and excitement pounces into action to inspire the audience to speak and has a mental timer inbuilt waiting to be triggered off. One of the many outcomes the speaker could face is not knowing when would be the appropriate or right time.
It would be too late and awkward to realize from the time the presenter asks for the crowd's participation and coming to the rude awakening that he is not getting the desired outcome. That would crush him or cause him to make further mistakes, creating a not so promising atmosphere and an uneventful presentation.
We can avoid these situations. Let me spark off some ideas that could alter the course of your presentation and making it not only interactive but interesting as well.
Some preparation that you could do is to make as many friends with the audience as possible. You could do it before your speech or during the speech complimenting your audience, possibly gaining some favour or eventually some fans. Just by making friends ensure you are not talking to a cold audience.
Observe bodies that speak
Look out for possible gestures and movements of your audience and pay closer attention to those who nod or smile and build on them. They are your anchors that illuminate your environment. They can possibly assist you in the interactive portion of your presentation.