Petrie & Ducky are creating scary shadows without the others knowing it is them.
Petrie: "They scared of Petrie! Petrie veeeeeeeeeery scary!"
Ducky: "Oh yes, yes, yes. You are, you are."
Petrie: "Me will eat you aliiiiiiiiiiive!" (chuckles evilly)
Ducky: "And then I will bury your gnawed bones, oh yes I will!"
Cera: "WHAT?! That-that were you two?!" (looks very angry)
Petrie: "H-how you know it me?"
Ducky: "I think we better run, yep, yep, yep!"
Cera: "Stay where you are so I can bury YOU!"
Petrie & Ducky: "Uh oh... AHHH!!!"
PS: Just ignore the fact that Petrie's cowering too on the screenshot actually... the scene is just too hillarious

Ignore Ducky's hands as well.