"You last saw Izzie flat-lining, so... I'm either there as a ghost 'on the other side,' or I survived a disease [stage IV metastatic melanoma] no one survives," Heigl said during a Monday visit to Late Show with David Letterman.

Video: Katherine Heigl shows some skin for In Style

When Letterman asked how she could be pulling 17-hour workdays — as Heigl had shared earlier in the sit-down — if Izzie isn't alive and kicking, she countered, "What if I was in bed in a coma? They could do that."

Heigl shared that it was "a little weird" reporting to the Grey's set last week without pal T.R. Knight around. (Last month, the actor, who played George O'Malley on the show, negotiated an early release from his contract with the ABC drama.) When asked for how much longer she will stick with Grey's, Heigl said with a trace of sarcasm, "That's what I keep wondering."

Heigl, who is contractually bound to Grey's for another two seasons, said that while she hopes the show enjoys the longevity of, say, ER, which ran 15 years, she'd like to continue playing Izzie "maybe not quite that long."

Heigl stars opposite Gerard Butler (300) in the romantic comedy The Ugly Truth, in theaters July 24.