David Labrava, who plays Happy Lowman, is a real life member of Hells Angel MC. He was hired as a technical advisor but asked Kurt Sutter for a chance to be on the show. He won't talk about his association with Hells Angels (though photos of him wearing his kutte can be found online and he has the death head tatted on him). It is said he's currently an inactive member since doing the show, but is still in good standing with the club.
David Labrava's (Happy) tattoos are all his own even the smiley face on his arm which is the reasoning for his character's name.
During his brief guest appearance in one episode, Stephen King played a character called Bachman. The name comes from a pen name, Richard Bachman, that Stephen King used early on in his career so that he could publish a large quantity of books without diluting the Stephen King brand.
The tattoo artist Irish Freddy from season two is played by real life tattoo artist Freddy Corbin. Freddy Corbin helped Kurt Sutter design all the tattoos for the show.