I always do a read through - and I do one (sometimes two) before
I cast the film. I'll gather a few actor friends, give the script to them
a few days before the read through and deliberately not tell them
what my intent is as the writer or the director. I learn a lot by listening
to their interpretation of the dialogue.
I have completely rewritten characters that didn't play well because
of what I heard at a read through.
After the movie is cast I like to do at least two full read throughs to
get ideas from the actors about the characters they are about play.
That, too, has been quite valuable to me as a writer and director.