When it comes to the world of "Flash" and "Arrow," death sometimes doesn't always remain permanent. It's a classic comic book trope that a character that dies will inevitably return. And adding time travel to the mix would seem to make death entirely meaningless -- but ironically enough, when it comes to the time travel-centric "Flash" and "Arrow" spin-off "Legends of Tomorrow," Executive Producer Phil Klemmer said that in this case "death means death."

During an interview with Collider, Klemmer explained the rules of time travel for the upcoming mid-season series.

"We do construct a sort of rule on our show," he said. "If you kill me right now with a hatchet, the rest of the team on the show can't go back in time and stop you from hitting me with a hatchet. The rule is that you can't interfere with events that that you participated in. That creates a time paradox and black hole and horrible things happen.

We want to keep those tensions alive so when our characters are in moments of mortal peril that they do have consequences -- that it's not like 'Groundhog Day' and we'll start over tomorrow."

"DC's Legends of Tomorrow" spins off from "The Flash" and "Arrow" and is slated for a mid-season debut on The CW.