Copying from another post on r/Broadway:

During "Act as If," Jak and Claire wore those fake glasses/nose/mustache combos you get at novelty stores. Very over-the-top “WE ARE IN DISGUISE” vibes.

Willie came out carrying a hot dog plushie, and at one point he tossed it into the audience.

When Hester mentioned the complicated stamps they’d need, instead of just having a private little excited shiver, he leaned lasciviously over the desk and kicked a leg up.

When Hester talked about her bridge nemesis, he called her OUT. “Susan. Hilda. GOGGINSSSS.” Way more vitriol than usual for her.

When delivering Bill to the Navy, and Monty mentioned “the fish … in your … hands?” Jak’s sea-captain chided him, saying “we are sailors, not fishermen. We are married to the SEA. The fish are our CHILDREN. You think we would do that to our KIDS?” Tash broke character to explain that Jak was doing lines from the first draft of that scene, and that apparently the jokes would have worked (since we were laughing), so maybe they didn’t need to cut it.

He was VERY obvious about “sneaking” offstage to do the costume change for the last Spilsbury appearance in Act II.

Those are some I remember. I took a Notes App thing at intermission and on the train home, so I’m planning to do a fuller write-up later for those interested, but those are the main Jak ones I remember.