Rehearsing individual characters can be a plus. In the beginning, it is best to work with crowds on separate nights from the main characters. Main character rehearsing can become boring to those in the crowd scenes. There is no need to waste the people's or director's time. Directors will find themselves spending lots of the rehearsal time trying to keep the crowd members quiet during main character development.
Tech rehearsals are very important.
These are very annoying to the performers as there are so many adjustments to be made. Long before a tech rehearsal, the director should sit down with the cast and explain that they will be in their costumes and make-up for a long while that evening. They should be informed to be patient and courteous to the tech crew. These rehearsals cannot be avoided if you want to achieve the best look and sound. Dress rehearsals should run straight through with the director taking notes.