In a normal compartment car, the seats were located on the sides of the compartments and seats were not fully visible from outside. Some models have molded plastic compartment windows, making it harder to see into the compartment from the aisle side.
For the best visibility of passengers, I would look to open seating coaches, which have rows of seats rather than compartments.
For a deluxe experience, you can include either the DB (TEE) Panorama coach (1962-1990) or the SBB Panorama coach (1993-today) depending on the consist.

Regards

Mike C

If the goal is to have a train (any train) with passengers in it that can be seen from standing next to the track as the train goes by you need 2 things :
1) The passengers need to be installed - which is often a case of cutting bodies at the waste and gluing the upper part to the seats, depending on the design of the seats.
2) Having overhead lighting inside the car that illuminates the people (the exceptions are the cars/locos with large windows like the Gläserne Zug or Panorama Wagen like ones).

Depending on what you want, you can get nice Preiser figures to get a mix if unique and higher detail figures, or an eBay purchase bulk of cheap China sourced figures that you might add paint to to get variety in color scheme for a few $.

For the lighting, you can go with either the purpose sold lighting or bits of LED strips, where you have better control of placing the LED over the passengers in the seats and compartments.

I have many passengers standing looking out a window. Makes then easy to see but you still need to remove the feet etc.

Is there a Marklin bilevel car set that isn't electric?