I don't speak up much in lectures. That's not because I'm shy (well, not mainly). Nor is it because I have nothing to say. No, I'd say that the main reason is lack of opportunity.
I should start off by explaining that the British lecture system does not encourage participation in lectures. Unlike the American system (I'm told), you're being lectured at, and 90% of the lecturers expect nothing more than polite attention from you. Very rarely are you actively being asked to participate.
That leaves only two opportunities for speaking up: Answering questions that the lecturer asks and asking your own. And to be fair, a lot of people do answer and ask questions. But I have a problem with both.
When it comes to answering questions, the problem is the sort of questions that get asked. Some of them are too hard: I've never been very good at thinking on my feet, so I seldom work out the answer to a hard question in a lecture. Most of the others are too easy: If the answer can be found one line further down on the slide, then I'm not going to bother to tell it to him.
Asking questions presents a different problem. I've found out over the years that I can find out by myself most of the questions that I would ask in the lecture if I look over the lecture notes later on. That knowledge, and a bit of pride, prevent me from asking them during the lecture.
I realise that all of these problems could be solved by changing my attitude, but it doesn't seem worth the effort. I don't get the feeling that more participation will give me a deeper understanding of the material. It just seems like so much wasted effort. And maybe that's one of the flaws of the British system.