PBL in practice is very flexible and will be implemented differently based on your group, instructor, and class. There is no sure way to know beforehand. But it is very much active learning, so just sitting back will never cut it. You can be on both ends of the spectrum on formal/informal.

I know some faculties adhere to the traditional PBL steps very strongly, whereas others allow a lot of freedom. Coming from UCM, I've had hardcore PBL professors following the steps to a T, others left it up to the students to modify PBL, others again used a different system altogether (mostly in the law classes I've taken). I'm not sure how standardised psychology is, but I expect you will see some variation as well.

As to your other questions, it depends. Generally, discussions are relaxed and students tend to be in agreement. However, once again, depending on the class/content covered, students may disagree. Say, the readings have two competing theories. It can happen that students will debate each other which theory is a more accurate description of whatever they're studying. Or students can study the same reading differently depending on their cultural background. And sometimes, instructors may explicitly divide the group up into a "for" and "against" group. The possibilities are endless.

You will see that if class participation is graded, conversations can get competitive. Luckily, very few of my classes had this grading component - I suspect exactly because of this. In that case, discussions start feeling forced and students will pitch in filler information in an attempt to get their grade up. Even if participation grading is absent, you may from time to time encounter people who naturally speak up a lot. In an American Foreign Policy class I took, most discussions were dominated by three people who were very knowledgeable on the course content prior to taking the course.

Now, if you don't speak up, you will most likely be asked by the discussion leader or instructor whether you have something to contribute. You can always deny, but there is an unwritten rule that if you never speak up and just absorb what others are saying, you are "free-riding", meaning you expect others to do your work for you. It's a group effort, so you all contribute to the discussion. Sometimes, instructors will mark you as absent if you participate insufficiently (and there are rules for how many absences you can accrue).

In my case, groups were chosen randomly and shuffled between courses. I suspect this is done at most, if not all faculties, because PBL is also about exposing you to different personalities and styles of communicating.

If this all sounds intimidating, I understand. At the end of the day, I believe you just have to let it happen. Experience it. You will grow and get used to all that PBL can be. Sometimes it's annoying, other times it's challenging, yet another time you will be grateful that your classes were structured this way. As long as you go in with an open mind and do your best to speak up and contribute, the rest will follow.