Male Thick-billed Vireos generally sing or chatter in response to hearing an intruder in his territory, though some approach silently. Females sometimes chatter in response to intruders, and sometimes approach as well. Males are very aggressive towards conspecific intruders as well as White-eyed Vireo (Vireo griseus) intruders; most attacked taxidermy mounts, and some ripped out feathers and removed eyeballs from the mounts. During agonistic interactions, the male often puffs out his chest feathers, attains an exaggerated upright posture, and spreads his tail feathers, especially if his mate is present. When agitated and searching for an intruder, they flick their wings.
Thick-billed Vireos are very bold, and will often approach people within a couple of meters. One individual, after being caught, banded, and measured, perched on the author's finger, staying even while being filmed.