Unless you're quite sure adding one person at a time is not going to drive you insane, I suggest you set Show people added from movie credits at Preferences - People. Then people will be added automatically when the movie is added or updated. I further recommend setting Download full credits and From main page only at Preferences - Plugins (IMDb) - Configure. This will limit the number of actors to the first 15 credited (those allowed on the IMDb "main page").
When people are added from movies, no information is downloaded. You still have to do that manually. That can be done as needed or periodically by grouping or sorting people by Date added to isolate those most likely needing a download, or by Last modified to see which ones may be due for an update.