Sam Rockwell Drops F-Bomb in 'SNL' Debut

Sam Rockwell's vulgar ad-lib on'SNL has Twitter buzzing

Sam Rockwell's vulgar ad-lib on'SNL has Twitter buzzing

Golden Globe victor Sam Rockwell made a big splash while guest-hosting "SNL" this weekend - and he did it in a way that wasn't scripted!

During a sketch called The Science Room With Mister Science, Rockwell plays a friendly scientist who is driven slowly mad by two incredibly dense (and not in a scientific way) students. At which point Rockwell starts losing his temper, and replied, "This isn't a true or false Josh, you can't be this fucking stupid!"

You can see him break character for just a split second when he realises what he's done (which was apparently bleeped for the LA broadcast, but not for the NY one), but quickly gets back into character, once again screaming at the kids in the room.

Rockwell presumably noticed his mistake, putting his hand briefly over his mouth. At which point the kids interrupt with, "Back to the Future?".

The 49-year-old Three Billboards Outside Ebbing, Missouri actor appeared on The Tonight Show Starring Jimmy Fallon on Thursday night (January 11).

He will also be accompanied by first-time musical guest Halsey.