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.
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.
Charlie Hunnam, playing Jax Teller has one of the highest kill counts of the entire series, with 45 direct murders by the finale, 26 of which were named. In addition to several unnamed members of the gangs of Charming (Niners, Aryans, Lin Triad, Lobos Sonora cartel, etc.).
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.
Kurt Sutter has stated several times that the homeless woman was always meant to be ambiguous and that the audience can make up their own interpretation for her. Conflicting information has been found that Kurt Sutter said he wrote the character as an embodiment of Jesus Christ, but tried to leave it ambiguous. It's still open to personal interpretation of each individual.
Walton Goggins' role came about because he read an interview where creator Kurt Sutter explained why Michael Chiklis and Goggins hadn't guest starred on the show like many actors from The Shield. Sutter explanation was that their characters were too associated with the actors and audience would think about Vic and Shane seeing Chiklis and Goggins. Goggins then called Sutter and, to prove him wrong, he suggested playing a transgender character.
In the episode where Gemma and the Sons are at her father's house in Oregon, Tig offhandedly mentions to Gemma, played by Katey Sagal, (who is in hiding as a fugitive due to a federal warrant for her arrest) that in order to go unnoticed maybe she should go redhead for awhile. Gemma then states, disgustedly, that she would rather shave her head than dye it red. This is most likely a tongue-in-cheek reference to Katey Sagal's character Peg Bundy from her show Married with Children who was known for her large bouffant red hair.
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.
With Michael Chiklis appearing in one of the last episodes of SOA it completes appearances of the original 4 members of the Strike Team from TV's The Shield. Vic- Michael Chiklis, Shane- Walton Goggins, Ronnie- David Rees Snell, Lem- Kenny Johnston. Not to mention many appearances from main characters from the Shield, including Dutch- Jay Karnes, Acevada- Benito Martinez, Claudette- CCH Pounder, Billings- David Marciano.
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.
While researching his role, Charlie Hunnam came to realize that the riders he was with rarely wore boots. They actually wore sneakers. You can see throughout the series Hunnam wears white sneakers, except to throw bloody ones away and trade them for boots near the end.
Despite numerous references and a large chunk of actors from The Shield appearing on this show, Katey Sagal is the only main cast member to appear on both shows. Everyone else that appeared on both were only "guest stars" on SoA. Including creator Kurt Sutter.
Like David Labrava, another member of the real life motorcycle club Hell's Angels appears on the show. Sonny Barger, one of the founding members of the Oakland chapter of the HA makes several cameo appearances as Lenny "The Pimp" Janowitz.
There are actually 4 real life Hells Angels members to have parts in the seriers. "Happy" was played by real life H.A. member David Labrava. "Quinn" is a real life Angels member Rusty Coons. "Lenny the Pimp" is played by real life Angels member and long time President, Sonny Barger. And "Frankie Diamonds" is played by real life Hells Angels member Chuck Zito.
Walton Goggins plays the Venus Van Dam in several episodes, this character's name is clearly a play on Cletus Van Dam, which was an alias his character in "The Shield" amusingly used on more then one occasion.
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.
Chibs and Tig are the only SAMCRO members to survive from the pilot episode to the end of the finale. Happy also survives the series from the pilot episode, but he was originally a member of the Tacoma charter, then he went Nomad, then finally joined SAMCRO in season 3.
Kurt Sutter had stated that he never planned to have Walton Goggins or Michael Chiklis on the show, simply because their performances/characters from The Shield would overshadow any role they had on SoA. Goggins repeatedly requested a guest appearance, to which Sutter finally replied "The only way I'll have you on this show is if you've got tits and a wig." to which Goggins replied "...alright let's do it!". As for Chiklis, Sutter decided to give him a small, but significant role in the final two episodes as the man driving the Semi that Jax crashes into, because he thought it would be poetic to have the main character from The Shield literally colliding with the main character from SoA. As both characters were the centerpiece of so much violence and death.
Marcus Alvarez is the only enemy of SAMCRO to survive from the first season to the end of the series. Though he did form an uneasy alliance with SAMCRO for later seasons. Ethan Zobelle was the main villain in season 2 and he managed to escape. Darby was also a villain in the first two seasons, but he reformed his ways and is only seen twice more in seasons 3 and 6.
Jax makes a comment to Wendy in Season 1, saying something along the lines of, "Don't worry, I won't go all Adriana on you," referencing Drea de Matteo's character and final demise in The Sopranos. He also says this again to luanne when he tells her about becoming partners in her porn business, she runs scared of him and he asks her " you thought I was going Adriana you?"