The Love Actually star, 64, has reprised the role of Daniel Cleaver for the upcoming movie, adapted from Helen Fielding's novel Bridget Jones: Mad About The Boy, which sees Bridget navigate life as a single mother after the death of her husband Mark Darcy.
Grant told the BBC's Graham Norton Show: "It is a good and moving script - it is extremely funny but very sad.
"There was no obvious role for me, but I was crammed in.
"So we didn't have a 60-year-old Daniel Cleaver wandering around looking at young girls, I made up a good interim story for him."
Daniel was a former love interest of Bridget, played by Renee Zellweger, and is the main antagonist in the film series.
Grant also spoke about his parents' view of his acting career, saying: "They were not at all supportive.
"My mother was a churchgoer and wanted me to be the Archbishop of Canterbury.
"In their world show business meant nothing.
"I remember when Four Weddings and a Funeral came out, my mother went to a dinner party with like-minded people and when asked about her two sons she said, 'One is an investment banker and the other is a film star'.
"Another guest said, 'How very interesting, which bank?' That's the world I grew up in."
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