I noticed that Mary's modelsheet includes mouth shape models, but Brian's doesn't. Mary has talked a few times, so it must have been planned ahead of time. I wonder whether if Alex or Maria's modelsheets have them, considering they both had several lines in season 1.
Well Brian never had any speaking roles in any of the episodes so far. Mary was heard speaking about four times that I'm aware of. Alex and Maria likely do. I can safely say Alex, Brian, and Maria at one time were suppose to be involved in the series at some point prior to the show airing in 1996 along with Luke, Frankie (Otis), and Patrick. There are some other unnamed third graders that were likely to be in the same position including Mary herself. Plans change. At least these characters made it into the Arthur universe to begin with.