Jon, Panel One: Childlike, questioning, possibly concerned about strange but certainly innocuous practices he does not understand, like "slide projection".
Liz, Panel One: Sweet, amused, slightly condescending in that approving way that only your mother or girlfriend is allowed to speak to you.
Jon, Panel Two: Though already excited for the lecture to begin, the promise of such minor embellishments as pictorial slides pushes the deal into entertainment territory for Jon. What sort of avant garde "shows" Jon is used to attending, I cannot say, though I do know he is easily shocked by exotic sights in motion pictures.
Liz, Panel Two: Knowing silence, as with all silences in Garfield, gives us special windows into characters. Anticipating the disaster of the next panel, and possibly just the refreshing company of a man thrilled by the prospect of slides.
Jon, Panel Three: Garfield Storytelling 101: Audience reaction is a funnier reveal than seeing a picture of a diseased intestine.
Liz, Panel Three: The moments when we can stomach repulsive sights our friends cannot is a small joy not documented often enough. Savor them. They bring us closer together, even when those moments include a woman's high tolerance for photos of rotten colons.
Recurring Gag: Jon has an unfortunate tendency to demonstrate willingness to participate in Liz's life by faking fascination with animals' excretory systems.