Cut Song: Several of them, though not all are listed here:
"Carousel." Quincy Jones had it cut because he didn't want the record to have two novelty songs (the other being the title track).
A Cover Version of "Behind the Mask", which was cut because of a disagreement about the royalties between Jackson's management and that of the original writers. It did get released in 2010's Michael, albeit in a somewhat remixed form, in part thanks to both the YMO and Jackson back-catalogs being owned by Sony Music (YMO through Alfa Records, Jackson through Epic, the latter of whom also owns the international rights to the YMO catalog).
"Got The Hots". Was completed, but left off of Thriller. However, Siedah Garrett, who later worked with Jackson on "I Just Can't Stop Loving You" and "Man in The Mirror", would record her own version of the song with completely different lyrics and a different title, "Baby's Got It Bad", for her album Kiss of Life. The Japanese version of Thriller's 25th anniversary reissue added the original demo as a bonus track.
"She's Trouble". Also know as just "Trouble". It was originally written with Jackson in focus, but was cut for unknown reasons. However, it would later be given to British reggae-pop group Musical Youth, where it would be released as a single from their second album, Different Style!
"State of Shock", a duet between MJ and Freddie Mercury. Wasn't completed in time for Thriller, and MJ wound up re-recording the song with his brothers and Mick Jagger for the Jacksons album Victory.
"Nite Line". Was rejected by Jackson, so Glen Ballard gave it to the Pointer Sisters instead.
"Hot Street". Originally known as Slapstick early on and was written by Rod Temperton.
Executive Meddling: A rare case where executive meddling actually benefited the artist in question. When "Billie Jean" was released as a single, MTV, then still in its infancy, and already notorious for rarely airing videos from black artists, refused to air the song's music video. CBS Records executive Walter Yetnikoff, Jackson's boss at the time, went ballistic upon finding out, and not only threatened to go public about the network's racial bias, but also completely pull all of CBS' music videos off the network unless the video was given airplay. This finally broke MTV's "color barrier", and kickstarted Jackson's runaway success on the network.
Walter Yetnikoff: "I'm pulling everything we have off the air [...] I'm not going to give you any more videos. And I'm going to go public and fucking tell them about the fact you don't want to play music by a black guy."
Follow the Leader: "Thriller" changed the music industry in many ways. By becoming the best-selling record of all time, all pop musicians, including Michael himself, tried in vain to surpass it. The use of innovative and amazing music videos to promote the singles also set a trend that is still felt in pop music today. If an artist wants his hit single to be commercially successful he is forced to make a music video for it. Of course, this has led many people to be far more interested in the visuals than the actual music, an evolution many musicians and music fans regret.
Long as the video is, it could've been even longer. As played on a re-release CD bonus, there was a second verse of Vincent Price's rap, ending in "Can you dig it?"
The title track was originally a love song called "Starlight".
Quincy Jones wanted to trim the intro to "Billie Jean," thinking it was too long. Jackson persuaded him not to by saying that the intro was what made him want to dance, and as Jones later admitted, when Michael Jackson tells you that something makes him want to dance, you don't argue.
Enough material was recorded for the album to potentially release it as a double-LP, and Jackson initially wished to release it as such once recording sessions had finished. However, producer Quincy Jones convinced Jackson to trim it down to a single disc, aware of the commercial unviability of double-albums in the wake of Fleetwood Mac's 1979 double-LP Tusk underselling compared to its wildly successful predecessor; Jackson, a notorious perfectionist, reportedly upset when he had to cut out an entire record's worth of material in post-production. Much of the scrapped songs would later resurface in leaks and officially as both bonus content on reissues and remixed tracks for posthumous studio albums.
Writing by the Seat of Your Pants: Owing to a scheduling snafu which wouldn't allow him to write it earlier that morning, Rod Temperton was forced to write Vincent Price's spoken-work portion of "Thriller" while waiting for his car to drive him to the recording session.