10cc in Strawberry Studios, 1975. From left: Lol Creme, Kevin Godley (at rear), Eric Stewart, Graham Gouldman. (pic from Sound on Sound)
From the Wikipedia entry:
A collaborative effort built around a title by Eric Stewart, "I'm Not in Love" is notable for its innovative production, especially its choral backing. An ethereal sound was created by laboriously building up multiple overdubs of the voices of Stewart, Graham Gouldman, Kevin Godley and Lol Creme singing a single note in unison. This multi-track was then mixed down and dubbed down onto 16-track tape. This process was repeated across all sixteen tracks to create a lush 256-voice "virtual" choir that could "sing" chromatic chords.
A number of these prepared multi-tracks were then cut into several endless loops, each which contained the basic notes of the main chords used in the song. The chorus loops could then be played by using the mixing desk rather like a keyboard -- each chord could be sounded by bringing up the fader for that loop. The instrumental break featured the repeated spoken phrase: "big boys don't cry...", which was spoken by Kathy Warren, the receptionist of the Strawberry Studios where the band recorded the track.
Strawberry Studios secretary Kathy Redfern, whose enthusiasm for "I'm Not In Love" helped to persuade the band not to scrap it, and who eventually supplied the whispered female vocal. (pic and caption from Sound on Sound)
In this pre-sampler period, the group was able to simulate a large polyphonic choir, creating a dramatic tonal effect similar to that produced by the well-known choir setting on the Mellotron, but with a far richer sound, and in full stereo. The same effect was used in Billy Joel's ballad "Just the Way You Are," released two years later. Released in May 1975, it became the band's second number one single and stayed on the top spot for two weeks from June 22nd.
Everything you could possibly want to know about the technical aspect of its recording can be found at Sound on Sound.
Here they are lip syncing to it on August 20, 1975, on "Musikladen" (sort of like the German "The Midnight Special"). Even though the performance is lip synced, they do it well, and it gives you a good idea of who is singing which of the background parts. The chromakeyed stars background is perfect, because since I first heard it, I've thought "I'm Not in Love" was sort of science fiction-flavored. That could be because the tinkling, twinkling sounds have always reminded me of those used by Vangelis on the soundtrack to Blade Runner (1982). Might Vangelis have been inspired by 10cc?