Neeson is reprising his role as widower Daniel while Brodie-Sangster is returning as Daniel's stepson Sam.
Neeson and Brodie-Sangster, now an adult after appearing in the 2003 film as a child, have revisited a scene from the original movie in which two characters had a conversation about the "total agony of being in love".
Sitting on bench along London's South Bank overlooking the River Thames, the actors slipped back into their old characters.
The short film, in aid of Comic Relief, will catch up with many of the other characters over a decade after their paths originally crossed.
It has been written by Love Actually's director Richard Curtis, who was on set on Thursday when filming kicked off.
Hugh Grant, Keira Knightley, Bill Nighy, Colin Firth, Rowan Atkinson and Martine McCutcheon are among the film's stars also making a comeback for the good cause.
Red Nose Day Actually will be broadcast during BBC One's live Red Nose Day 2017 programme on March 24.