Mars called into Ryan Seacrest's KIIS FM radio programme to discuss 'It Will Rain', which will appear on Breaking Dawn's soundtrack.
The singer hinted that 'It Will Rain' might also feature in one of the movie's most pivotal scenes.
Mars said: "I started writing ['It Will Rain'] a little bit on tour. I was just writing to write. I had my guitar with me on tour when I was in the States. I never finished it.
"The opportunity came up to go see some scenes in Breaking Dawn. When I saw a couple of the scenes I was like 'That song I started might be perfect for this'… I've seen a couple of things. I might have seen them get married."
The music video for 'It Will Rain' was to premiere in late October, but the clip's debut was later pushed back.
Mars apologised for the delay, but commented that he wants the music video to be perfect before it's seen by the public.
"I'm sorry about that. I take it very seriously. If it's not right I don't want to put it out," the musician said.
The Twilight Saga: Breaking Dawn - Part 1, which also features tracks by Christina Perri and Iron & Wine, is now available in the UK and is released on Tuesday (November 8) in the US.
Breaking Dawn - Part 1 is released in cinemas worldwide from November 18.
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Listen to Bruno Mars's track 'It Will Rain' below: