You may have heard of the English singer-songwriter and producer Nick Lowe, whose biggest hit was 1979's 'Cruel To Be Kind.'

Lowe wasn't just a one-hit wonder - he had been in a English 'pub-rock' band in the early-to-mid 1970s called Brinsley Schwarz, and their last album had one of his songs called 'What's So Funny About Peace, Love & Understanding?' In the late 1970s, Nick Lowe was the producer for another English singer-songwriter, Elvis Costello, and in 1978, Lowe needed a b-side for his single 'American Squirm', and he got Costello and his band to cover 'What's So Funny...'. Costello and his record company ended up liking the version of the song so much that they subsequently released it as a single of their own, having something of a hit in the UK with it.

Anyway, by 1993, Nick Lowe's career was not in the best place - it had been some time since 'Cruel To Be Kind', and he felt like he was fading into obscurity. In the story he told interviewers at the time, he looked at his balance at the ATM one day, and was astonished to see there was a million dollars in it.

He eventually figured out that the reason for this million dollars was that it was not a mistake: the blue eyed soul singer Curtis Stigers had covered his song 'What's So Funny...' on the Bodyguard soundtrack.

Which, as /u/CommodoreCoco makes very clear, is one of the biggest selling albums of all time - and so having the songwriting credit to an individual song on the soundtrack was worth a million dollars in of itself (Lowe used that million dollars to basically fund his subsequent career as a musician, changing his style and becoming a critical favourite on the 1994 album The Impossible Bird).

As to why The Bodyguard was so successful, the 1990s was an era of prominent film soundtrack themes - apart from Celine and Titanic, think Bryan Adams' 'Everything I Do, I Do It For You' (Robin Hood: Prince Of Thieves), Aerosmith's 'I Don't Want To Miss A Thing' (Armageddon), or Bryan Adams, Rod Stewart and Sting's 'All For One' (what was that three musketeers movie called again?), or Seal's 'Kiss From A Rose' (Batman Forever), or U2's 'Hold Me Thrill Me Kiss Me Kill Me' (also Batman Forever). This is not an exhaustive list either.

The 1990s was also an era where the music industry was making a large amount of money from CD sales - they were convenient, easier to stock than vinyl, more durable and better sound quality (well, there's an asterix there, but they were marketed as such), and this was an era before there were digital competitors such as Napster or Spotify. CD players, by the 1990s, were cheap enough and small enough that portable battery powered players became affordable and relatively ubiquitous. As a result, the 1990s was the glory years for the music industry in terms of profitability.

1992 was also a good time to release Whitney Houston music. In the US, she had three number one singles from her 1985 debut album, Whitney Houston, four number one singles from her 1987 album, Whitney, and a comparatively disappointing two number one singles from her 1990 album I'm Your Baby Tonight. Both Whitney and Whitney Houston have sold 'diamond' in the US, indicating sales of more than 10 million each.

So, from a purely mercenary record company perspective - and Whitney's record label Arista was run by Clive Davis, who is famously quite ruthless in the search for hits (having been sacked from Columbia Records in 1973 due to a payola scandal, for instance, and notorious these days for wanting Kelly Clarkson to have a different kind of career to the career she wanted to have), so that record company perspective makes sense - putting Whitney on a soundtrack CD was joining several popular things. Whitney! Songs in a movie! A CD!

(It should also be said that Clive Davis fairly ruthlessly sculpted Whitney Houston's public image into a very racially-neutral family-friendly image which she clearly did not enjoy and pushed back against unsuccessfully.)

Additionally, The Bodyguard is a film where Houston plays a popular singer, and it was a very popular film - according to wikipedia, which is usually accurate about basic facts like this, it made over $400 million dollars, and it was the second biggest film of the year after Aladdin.

The soundtrack to the film has two sides, more or less - the first side is new material by Whitney Houston, and the second side contains other music from the film by other artists (such as Curtis Stigers' version of the Nick Lowe song). In contrast, the soundtrack album for Titanic, apart from the Celine Dion single 'My Heart Will Go On', is orchestral film score music, which generally doesn't sell as well as pop music (but Titanic's soundtrack is, again according to Wikipedia which is usually accurate about these things, the biggest selling orchestral film score ever).

Soundtrack albums to popular films - such as The Bodyguard have some advantages over other albums: namely, that the music is likely already familiar to millions; while record stores had booths where you could listen to music, they were not 'try before you buy' quite like a record is today (when anyone can download the album illegally, or listen to the record on some morally dodgy but legal streaming service), and so there was a sense of stepping into the unknown when buying a new CD, which made some buyers hesitate. However, if you'd already heard the songs in the film, there was less uncertainty - you knew what you were getting - and The Bodyguard didn't just have music as background music, it was integral to the story. And not only the music was integral to the story, but Whitney Houston - the singer of about half the songs on the record - was the star of the film. And one of the biggest stars in R&B-flavoured pop music at the time. Who sang half the songs on the album.

Further to that, as /u/CommodoreCoCo points out, 'I Will Always Love You' in of itself became one of the biggest songs of the year, and 'I Have Nothing' and 'I'm Every Woman' were decent sized hits (both #4s) that received plenty of pop airplay, and which probably would have been more successful as singles had Arista promoted them as singles rather than promoted the album. This would have meant that people who missed out on seeing the film during its run in cinemas might still have bought the album, thinking, well, I like all the singles.

So, ultimately, I don't think there was any particular need for innovative promotional ideas or extensive hype here. It'd be something like if Taylor Swift had been cast in the next Marvel film as a singer who was secretly a superhero, and there were definitely three songs on the soundtrack that pretty obviously sounded like guaranteed Taylor Swift hits on the soundtrack. That she prominently sang in the movie, maybe more than once. And where she was basically co-lead with one of the biggest stars of the franchise - Chris Hemsworth/Pratt? Tom Holland? And it was one of the biggest films of the year. Radio stations and streaming services will instantly add that music to their playlists, there'd be memes about the songs on /r/MarvelMemes, everyday people who don't usually pay that much attention to modern pop music (as opposed to people who know what /r/popheads is) will notice the songs, etc.

Which is to say, the reason for the success of The Bodyguard soundtrack was just a lot of already popular things coming together in the one package. I suspect Clive Davis probably used the word 'synergy' a lot in 1992, but in his memoir Soundtrack Of My Life, his main claim to having personal responsibility for that soundtrack's success is that he chose an early and quite unadorned mix of 'I Will Always Love You' as the final version, over furious objections from the song's producer, David Foster. But his memoir is in any case more focused on his connections to famous people rather than explaining the details of record company retail strategies.