It depends on the lecturer, how many people there are in the lecture, and whether they know your names!
One of my lecturers always picks on people, especially if they look like they are purposefully averting their eyes, as she's learnt all of our names just so she can call on anyone at any time (granted, our lectures are always pretty small). In seminars we are always expected to say something, regardless of the lecturer, even if it's just a one-word answer!
I know someone studying Maths though, who is in lectures of about 250, and he always gets picked on, because for some reason his lecturer remembers his name, and can't be bothered to ask anyone else!