The two actors starred in the 2003 Christmas romcom directed by Richard Curtis.
In a preview for the special, Thompson recalls the first time that she and Grant watched the finished film.
While the viewer response to Love Actually was positive, the Notting Hill star’s reaction was less predictable.
“Hugh came up behind me as we were walking out and said, ‘Is that the most psychotic thing we’ve ever been in?’” Thompson said.
Grant appeared not to remember their interaction, responding: “Did I say that?”
In the hit movie, Grant played the Prime Minister who falls for a Downing Street staff member Natalie, played by Martine McCutcheon.
The role involved a solo dance sequence to the Pointer Sisters’ “Jump (For My Love)”, which the actor has previously said was “absolute hell” to film.
Earlier this month, Nighy – who played ageing rocker Billy Mack – revealed that the one iconic Love Actually quote spoken by his character that he thinks will be at the top of his obituary.
Diane Sawyer’s special – titled The Laughter & Secrets of Love Actually, 20 Years Later – premieres on Tuesday (29 November) on ABC.
The special will then be made available to stream on Hulu. A UK release date is yet to be announced.