If you're a poor grad student like myself, you know owning fancy tools like a light box is a luxury. Hopefully your university should have one or two for use (mine does), but if you are at home rendering and realize you need a duplicate character, here is my hack for a light box.
Make sure your original and "new copy" are lined up. Tape the corners to the window very lightly. Trace away!
Needless to say, make sure it's light outside. Don't try this at night unless you live in my apartment with a street lamp right outside the window. Even then, don't do it at night. Daytime tracing for best results.
This really doesn't replace a light box or scanning your design and printing another. But, if you need it on paper that doesn't work in your printer, this will work in a pinch.
If you're exploring designing a show but don't want to sketch out all the characters, check out the article: Interpreting Design Without Drawing.
The cheeky answer would be "as many times as it takes". But I'm not cheeky today.
Five could be the magic number. For the purposes of this post five is, but take it as a guideline, not a rule. If it is a play I've designed before I skip straight to #2 or #3. After meeting with the design team and director reading the play with their ideas and vision in mind also allows things to come forward that you may have overlooked the first few reads. But, when it doubt read it again, with a pad of paper and pen by your side.
Yes, I have written about reading the script before in one of my Life Lessons from the Costume Shop and the number of times and order has changed since I've written that post. These are guidelines as I stated above. Read both, see which you prefer.
Once you've read your play, check out this article on places to do research other than the internet!