All such data is added at entry level (tv or movie). So, find the episode or movie where the data is missing and add it there.

Excellent, thank you. The other question I had is that the listing for the cast appear as "regulars". Is there a spot for guest appearances? I am trying to update the new Twin Peaks cast and there is a lot of actors that appear in a few episodes but not what would be considered regulars. I am asking now because I do not want to add these actors in only to have them deleted.

Hi, guest stars (here defined as anyone not a season regular) should be added for each episode. Here is a starting point (episode 1) where you can get going. Just click on Guest Stars and start adding them. Then you can get to the next episode by clicking just below the content score. (Edit: Or just by changing the URL if you feel that is quicker.)

Note that guest stars can not be sorted at this time. I usually start with the notable ones, but sometimes they jump out of order anyhow.