Well, not quite. Guest Lecturers often exist in a "grey area" as such: not full passengers not crew either. Although they will normally be on the Passenger List (but not always) exactly how they are treated depends a lot on the individual management on that particular ship and on the Lecturer themselves.

I've known Lecturers who have had the best of both worlds - well respected and treated as part of the crew "family" whilst being a popular passenger, but also the opposite that they've alienated the crew with their attitude and ended up not getting treated as well as they could have been.

What PeterBruce says is correct though - be prepared for very mixed treatment in regards to scheduling - and again, it's not just by the line but due to the management on a particular ship at the time. Some are amazingly organized and will make themselves available for you to speak to about any issues and will work out your schedule with you; others will leave you to find out you have a lecture only from the passenger daily events paper the night before and actively avoid dealing with you even when they know you need to speak to them.