According to an interview with Kurt Sutter, the pilot episode was entirely shot with actor Scott Glenn in the role of Clay Morrow. After deciding to go in a different direction with the character, the role was re-cast with Ron Perlman and Clay's scenes were re-shot. Also, in the pilot there is still a scene where you can spot Scott Glenn as Clay, after they blow up the warehouse at the end of the episode there is a shot of the Sons running away from the explosion. Scott Glenn can be seen in the foreground and you can also see Emilio Rivera in the group as he was originally cast as a member of SAMCRO but then re-cast as the leader of their rival gang; The Mayans.
David Labrava who plays Happy Lowman is a real life member of Hell's 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 Hell's Angels (photos of him wearing his kutte can be found online, he also has the death head tatted on him), but its said he's currently an inactive member since doing the show, but is still in good standing with the club.
When the series explores the Sons' deeper involvement with the IRA and ultimate trip to Belfast, Ireland, the episodes chronicling the chapter's period overseas are titled in Irish: "Lochan Mor" (Big Pond/Lake) (S3/E8), "Turas" (Journey/Tour) (S3/E9), "Fírinne" (Truth) (S3/E10), "Bainne" (Milk) (S3/E11). The theme song "This Life", that opens every series episode, features the addition of Irish traditional musical instruments (flutes, tin whistles) for the duration of the Irish stopover. Episode 13 (season 2) is titled "Na Trioblóidí" which is Irish for "The Troubles", a period generally pertaining to the 30-year political/religious unrest in Northern Ireland that ultimately ended with the Good Friday Agreement in 1998.
The phone tapping device used by the Federal agents in season 2 is in actuality Pro-Tools software running off a laptop and connected to an M-Box interface. In real life it's used for audio recording and editing, not wire tapping.
The trivia items below may give away important plot points.
Kurt Sutter revealed in an interview that he realized Tara had to die when shooting season 2. The reason for this was that when facing the final season, Jax had to be a man completely devastated, without supports.