There are some theories that our lovable Jack Falstaff–you know,
sweet Jack Falstaff, kind Jack Falstaff, true Jack Falstaff, valiant Jack Falstaff, and therefore more valiant, being, as he is, old Jack Falstaff
–was not originally John Falstaff in the first productions of The First Part of Henry the Fourth. Instead, he was John Oldcastle. In fact, there remains a line in the play that seems to point to this: in Act One, Scene Two, near the beginning of our introduction to the white-bearded Satan, Hal refers to him as “my old lad of the castle” (I.ii.41-42). Not only does this use both the words “old” and “castle” (Oldcastle), but the reference to “the castle” is important as well. The Castle was a noted Eastcheap brothel, thus, setting up the notion of the character’s disreputable reputation.
So, why did the Shakespeare make the change?
The theory is that John Oldcastle was a reference to a historical figure of the same name, who had fought for Henry IV against Glendower, and who had become a friend of Hal (Henry V). Oldcastle had begun to hold a slightly reformist religious view called Lollardy, and because of this he was arrested for heresy and excommunicated before the death of Henry IV. When Hal became king, he gave Oldcastle a respite during which he escaped. In his flight, he became a major part of a Lollard plan to kidnap Henry V, and later knew of the Southampton Plot. He was later captured and executed, and became a martyr of sorts to anti-Catholics. When Shakespeare wrote his play (after the fall of Catholicism in England), Oldcastle’s descendents, including Lord Cobham who was Lord Chamberlain at the time, became so angry that Shakespeare was forced to make the change.
Since the name Falstaff (and thus, theoretically, Oldcastle) never appears in any verse of the play (only in stage directions and prose), Shakespeare was free to replace the name at will and with ease. But why “Falstaff”?
There was another Lollard, John Fastolf, who fought in the Hundred Years War. While he was a soldier under Henry V, he served with such dignity that he was knighted. A few years later, however, he was involved in the English defeat at Patay. While many historians put the blame of the defeat on Lord Talbot and his rashness (remember him from The First Part of Henry the Sixth?), Fastolf became a scapegoat, since we was seen fleeing the battle after it had gone south; his reputation was so tarnished that even the followers of Cade’s Rebellion (remember The SECOND Part of Henry the Sixth?) charged him with the loss of Normandy to France.
A couple of letter changes (Fastolf to Falstaff) and Shakespeare throws an already scapegoated historical figure under the anachronistic bus, while rescuing John Oldcastle.