Walken was starring in Stephen Merchant's The Outlaws, which is a comedy-drama series about a rag-tag band of misfits doing community service, when he painted over the Banksy artwork.
You see, the show is set in Bristol, which is Merchant's home town, but also the home town of the elusive street artist.
In the last episode of the series - apologies if you haven't seen it yet, but this won't spoil too much - Walken and other members of the cast can be seen painting over a wall covered in graffiti.
Walken - playing a character called Frank - then came across a stencilled rat standing on a spray can that was signed by Banksy covered up behind a board.
He asked his probation officer whether he should cover that over too as it's 'awfully good', but received the response: "Less debating, more painting."
So, he painted over it.
Now, a spokesperson for the BBC has confirmed that the painting was actually a real Banksy.
A statement read: "We can confirm that the artwork at the end of The Outlaws was an original Banksy, and that Christopher Walken painted over that artwork during the filming of this scene, ultimately destroying it."
Of course, this is very much Banksy's style, and probably the kind of thing he - or they, perhaps? - might get right behind.
However, it doesn't mean that they didn't make a costly decision there, though.
Banksy's works, whilst daubed on walls around the world for the most part, have fetched serious sums at auction in the past.
His infamous piece of (sort of) performance art entitled Love In The Bin went for £16 million at auction last month.
That piece was originally meant to be sold as a picture, but as the sale was conducted it shredded through the bottom of the canvas, ruining the original print.
Of course, ruined is maybe not the word here, given the amount of value that seems to have been added.
Back in September, another piece that was painted on the wall of a shop in Suffolk added £200,000 to the asking price of the building.
Basically, there's good money to be had in scribbling on walls, provided you're Banksy.
Still, getting one of your artworks destroyed by Christopher Walken must feel pretty good, even for a successful artist.