Following the internet threats issued to Letterman from a Muslim extremist, Ashton put his safety first by donning a helmet, bulletproof vest and a sign that read "Not Dave."
"You can't be too careful sitting next to a guy that's been fatwa-ed," Ashton said through a wide grin.
Ashton soon got more comfortable on Letterman's couch, shedding his anti-fatwa gear and delving into his expectations as to how audiences will receive him as the new star of Two and a Half Men.
"I think it's going to go through the roof," said Ashton. "I've been laughing really hard."
You can watch Ashton's entire interview tonight on Late Show with David Letterman on CBS and Two and a Half Men returns to the eye network September 19.