My BritRail pass has the names of all the travelers in my group and spaces for their passport numbers. Do I need to have that filled out in advance?
I would say yes. One less thing to worry about.
You need to have the paperwork done before your pass is checked.
I guess you have a choice - you could calmly put the numbers in at home at a table, or you could write on your knee on the train and try to get legible numbers before the conductor reaches your seat.
Both work.