I think a lot about this post by Mary Beard, about how important it is to go back to primary sources, and a good thing as my first piece of proper graduate work turned up a solid first-hand example!
I was writing a paper looking at 15th-century versions of Judith, the biblical-apocryphal heroine who saved Israel by duping the enemy general Holofernes and cutting off his head. In France, Judith became connected with Joan of Arc, in the theme of sword-wielding female national liberators, and I wanted to look at whether that carried over to versions of Judith’s story in England.
A 2010 book on “Judith studies”, The Sword of Judith, pointed me to The Story of Judith in German and English Literature, a 1927 book-length bibliography of Judith adaptations. (In the olden days you had to do that sort of thing by hand…) That bibliography lists a lost play called Holophernes that was supposedly performed in 1556 for Princess Elizabeth (the future Elizabeth I) when she was under house arrest during her sister Mary I’s reign. The source quoted is a c. 1560s manuscript that describes Elizabeth’s guardian, the minor Catholic nobleman Sir Thomas Pope, paying for the play to be performed for her.