I didnt even think of Degaton! His title was technically "lab assistant" but he was a glorified janitor, wasn't he? Anyway, thanks for the segue, because I have a janitor-themed question now!

Name two DC Comics characters who have been voiced in animation by an actor best known for playing the ghost of a murdered janitor.

Man, what sorry SOB would murder a janitor?

And what would drive a janitor's spirit to walk the earth, never knowing the rest of the grave?

--that's just cold.
wyld

Maybe he died while mopping the floor and wasn't finished as he was killed. And just to spite the janitor, the criminal walked all over the newly mopped part of the floor with his muddy boots.
And now, since he's a ghost, he can't hold a mop, and so he's doomed to walk those floors forever more, never being able to finish his job.

Only popular janitor I know is Neil Flynn.

I questioned myself with Degaton, but given that he's generally seen with a mop, I figured it was OK.

He carries the mop to threaten people with. "I'll mop you off the floor". That's opposed to his other trademarked catch phrase, used when he catches someone unaware. That phrase being "How do you like them apples?". Then there was that he was down and out and had to make ends meet. The catch phrase he had then was, "Five dolla, me show you good time soldier boy". And, of course, who could forget, "shine your shoes for a nickel, mister?". Then, when he's escaping, he says "twenty-three skadoo".

That explains why he killed that guy who said "Go get your ####ing shine box!"

I may be oversimplifying by calling the character the "ghost of a murdered janitor". He was definitely a janitor, and he was definitely murdered, but what he became after death may be classified as a "ghost" or...something else. I don't think it's ever been clearly defined what he is.

The murdered janitor isn't a DC character; the actor best known for playing the murdered janitor did the voices of two DC characters in animation.

I amcompletely stumped on that.

My favorite actor is James Cagney, and I don't believe he ever voiced any DC character in animation. (But wouldn't he have made a great Rupert Thorne?)

Cagney as Rupert... that would be pretty good. Then again, after watching "White Heat", I think he'd of been a great "Joker", if it weren't for his height.
I wouldn't say Cagney was my fave actor, but he's on my top ten list. Mainly because he was so versatile in his roles... but, yeah, White Heat makes me think of the Joker as he was in the golden age.