What are the major differences with Falstaff in Henry IV, Part I versus Falstaff in Henry IV, Part II?
The central difference between Falstaff in these two plays is a result of the impact that Henry's healing of his relationship with his father has on his relationship with Falstaff. This occurs at the end of King Henry IV, Part 1, and as a result Henry puts into place his plan, revealed in secret to the audience, of withdrawing his friendship from Falstaff and his cronies. This explains why Falstaff appears to be so diminished in the second part of this play. In the first, he is the life and soul of the party, incredibly witty, and a delightful character. Note, for example, how in Act II scene 5, when he and the youthful Harry play a game of roleplaying to help Harry prepare for his meeting with his father. Pretending to be the King, Falstaff mainly urges Harry to keep Falstaff close to him, excusing all of his faults one by one:
No, my good lord, banish Peto, banish Bardolph, banish Poins, but for sweet Jack Falstaff, kind Jack Falstaff, true Jack Falstaff, valiant Jack Falstaff, and therefore more valiant being, as he is, old Jack Falstaff,
Banish not him thy Harry’s company,
Banish not him thy Harry’s company.
Banish plump Jack, and banish all the world.
It is this element of playfulness and humour that makes Falstaff such a memorable character. Although this is still evident in the second play, the difference is the ending, which features Henry's rejection of Falstaff, leaves such a profound impression on the audience, as Falstaff is shamed publicly and hurt deeply by Henry's open rejection of him, saying "I know thee not, old man." Athough Falstaff is a rogue, he is above all else a lovable rogue, and this rejection makes it very hard for the audience not to feel sorry for him. Falstaff moves from being a figure of hilarity to being a figure of pity between the two plays.