'Flash' Star Grant Gustin -- Quick With A Joke ... Co-Stars Beware

Watch your back around "The Flash" star Grant Gustin ... from the looks of things, he's the class fast clown on set.

Gustin broke his boredom during a shoot in Vancouver by busting out the ole bunny ears ... at a security guard's expense. Sure, he could have been flashing (yeah, we know) the peace sign -- but the smirk on his face is a dead giveaway.