The "Lost" legacy isn't an easy thing to pinpoint. For six years, the ABC series has defied genres: Sci-fi? Mystery? Thriller? Romantic epic? It also has captivated viewers with its time-shifting storytelling (this week's episode harked back to ancient mysteries and biblical references) and its memorable characters, while setting a TV precedent for cast diversity and attracting a global virtual community of devotees.

But no matter what aspect of "Lost" draws viewers, there's one part of the show that transcends all of it, distinguishing it from its TV brethren: its beautiful, sweeping score, recorded every week with a live orchestra.

If you have ever wept, laughed or felt terrified while watching "Lost," chances are Oscar-Emmy-Grammy-winning composer Michael Giacchino had something to do with it.

"Sometimes you can be very manipulative with music, but in the case of 'Lost,' generally, I'm just giving you how I feel about it," said Giacchino, who won his first Oscar and his second Grammy this year for his score to "Up."

"It's because I like the show so much. So for me, it's about transferring my reaction to the show. If you are sitting there crying, that means I was sad too."

'THE GIACCHINO'

Thursday night, ABC bid farewell to "Lost" (the final episode airs Sunday) with a classy celebration at UCLA's Royce Hall, featuring Giacchino conducting a 47-member orchestra through the series' score, including the iconic "Life and Death," last used when Jin and Sun drowned in the May 4 episode, and "Oceanic 6," which he wrote when six of the castaways left the island at the end of the fourth season. Tickets sold out in 10 minutes. The proceeds went to the Colburn School of Performing Arts in Los Angeles.

What "Lost Live" presented, essentially, is "The Giacchino," a term of endearment "Lost" executive producers Damon Lindelof and Carlton Cuse coined in the first season and use as shorthand in their scripts to signal feelings they want to convey to the actors and the director.

"For Damon and I, music is such a huge part of 'Lost,' " said Cuse. "We literally write Michael's name into the script in various places where we want to convey a sense of emotion. We wanted the final event to celebrate the incredible collaboration and the reason 'Lost' is so successful, and we feel that Michael is such a huge part of that."

The composer himself has never seen "The Giacchino" note on a "Lost" script because he's never read one. For six years, he has preferred to watch the episodes one scene at a time, and respond to his feelings with his songwriting.

"I've never wanted to know what's coming because I feel like they wouldn't get my real reaction musically somehow," Giacchino said. "I'm a big fan of 'The Twilight Zone' series and 'Lost' felt like a modern-day 'Twilight Zone,' but more emotional. And I love that you get vested in the characters. I became quite obsessed with the show and the mythology and trying to understand it. So it was my way of being able to watch the show and work on it at the same time."

For similar reasons, Giacchino insisted on working with a live orchestra that includes trombones, a string section, a percussion group, harps and a piano.

"You're getting the soul and the emotion of those 42 people that are sitting out there in the orchestra," he said. "They put everything of themselves into the music, and you can hear that. It translates in a way that you just can't get with a synthesizer. For me, music is best when played by people and you get a connection you wouldn't get otherwise."

AN AID TO ACTOR

"Lost" actor Daniel Dae Kim, who is filming "The Killing Game" in Louisiana and appeared on "Lost Live" with fellow actors Jorge Garcia, Nestor Carbonell, Michael Emerson and Naveen Andrews, considers the "Lost" score to be one of the show's most lasting memories. "Just today, I had a very emotional scene to play and I listened to the music to put me in a particular place to be ready," Kim said on May 8. "The music of our show is as big a character as any of the rest of us."

Indeed, it's impossible to listen to the score of "Lost" and not associate its playful, mournful and apocalyptic qualities with the figures and events of the show.

"We've always talked about the central aspect of 'Lost' being character, character, character, and his music is so evocative of a certain moment or person in the show," Lindelof said. "If you close your eyes and play 30 seconds of one of Michael's themes, you'd know which character's theme that is."

The memories rushed in for Cuse and Lindelof during the show's last recording session on Friday.

"When we put the music to the words and the picture, it finished the creative work of the show, and as we sat there listening to Michael's music play over the scenes we've written and Jack Bender had directed, there was this sense of conclusion," Cuse said. "We just sat there emotionally overwhelmed, not only because of the emotion of the moment, but because of the power of Michael's music."