“Tell the truth boldly, whether it hurts or not. Never pander to weakness. If truth is too much for intelligent people and sweeps them away, let them go; the sooner the better.”

Bishop
Kristnihald undir Jökli (Under the Glacier/Christianity at Glacier) (1968)

Credited to Shaw in the lead in to the mockumentary C.S.A.: The Confederate States of America (2004) and other recent works, but this or slight variants of it are also sometimes attributed to W. C. Fields, Charlie Chaplin, and Oscar Wilde. It might possibly be derived from Shaw's statement in John Bull's Other Island (1907): "My way of joking is to tell the truth. It's the funniest joke in the world."
Another possibility is that it is derived from Shaw's characteristic of Mark Twain: "He has to put things in such a way as to make people who would otherwise hang him believe he is joking."
Variants:
If you are going to tell people the truth, you'd better make them laugh. Otherwise, they'll kill you.
If you're going to tell people the truth, you'd better make them laugh. Otherwise, they'll kill you.
Disputed

"Inside the Actors Studio," 2009 http://www.youtube.com/watch?v=sBBtYcK9Jb8

Source: Survivor

Source: His Dark Materials, The Subtle Knife (1997), Ch. 4 : Trepanning
Context: Lyra sighed; she had forgotten how roundabout Scholars could be. It was difficult to tell them the truth when a lie would have been so much easier for them to understand.

Quinn (ed), Faithful History: Essays On Writing Mormon History, p 103, fn 22

Source: The Virgin Suicides