When Jean suggests Berenger see "one of those absurdist Ionesco plays", he holds up a brochure for that very production.
Many of Berenger's reactions in the first and second acts.
The way he doesn't react when a rhinoceros comes charging through town. He just sits and yawns.
His goofy smile when he greets Mr. Papillon.
His annoyed expression at another one of Botard's rants.
In Act II Scene I, everyone is arguing about the rhinoceros they saw and whether or not it exists. Enter bleary Berenger, not caring an ounce.
When the firewoman is helping everyone down after the staircase is destroyed, Berenger and Dudard can't agree on who should be rescued first, so they go down the fireman's ladder (read: a small stepladder) clinging to each other. The firewoman comments on how weird that is.