As the famous scenery-chewing actor lay on his deathbed, someone asked him how he felt about dying. “Dying’s easy,” he replied. “Comedy’s hard.” What actor allegedly said this?
I’m pretty sure Grissom said it in “CSI.” 
Also, Peter O’Toole used the line in “My Favorite Year.” I suspect it’s an old chestnut that has been around and recycled many times, whoever may have said it first.
The Quote Verifier agrees with Tim R. Mortiss.
Peter O’Toole says it in My Favorite Year
Oops. Sorry, Tim R. Mortiss; I should’ve read more carefully.
All of them.