Did Shakespeare ad lib?
If you are asking if Shakespeare himself ad libbed you would get
a resounding no. For Shakespeare was the author of the plays, the
actors themselves would need to be the ones guilty of an ad
lib.
I do, however, understand the question is asking if, in fact,
any of the Shakespearean actors ever used an ad lib.
This would be impossible to factually. An ad lib is something
added in to fill an entrance, exit or lengthy pause during a
performance and even used to cover if a fellow actor messes up or
forgets a line. Therefore there would be no proof anywhere to say
whether or not this occurred. The only way to tell is to compare
the performance to the script, since there were not recording
devices (obviously) we could never know.
I would like to believe Shakespearean actors did not ad lib.
Since his work is written in a specific rhythm it would be frowned
upon to ruin the flow of the piece and ad in things that were
unnecessary. I have been involved with Romeo and Juliet recently
and found that when an actor missed even the tiniest word in a line
the entire flow of the scene would get disturbed. I could see how
Shakespeare would greatly press onto his actors to say the lines
verbatim.