Together, written by Dennis Kelly and directed by Stephen Daldry, will follow a family from the first days of the Covid pandemic in March 2020 to the present day.
‘He’, played by X-Men and His Dark Materials star McAvoy, is a self-employed, self-made man who runs a boutique computing consultancy, but is forced to furlough his staff and take up growing vegetables.
Catastrophe star Horgan plays ‘She’, a Europe co-ordinator at a refugee charity and the daughter of a dentist father and an “old socialist” mother.
Arthur is their 10-year-old son and the one thing that has kept their relationship together until lockdown.
Glasgow-born McAvoy, 42, said: “Dennis writes so wittily for these characters and his script had me within the first few pages.
“When I heard Sharon was involved with Stephen Daldry directing it was a no brainer.
“It’s a hilarious, heartfelt, entertaining and unconsciously funny exploration of a relationship and a couple facing real tragedy across a year we have all lived through.”
Horgan added: “I think everyone who got involved in this film had no intention of doing a Covid drama but then we read Dennis’ script and changed our minds.
“We shot this in 10 days so it was a beautifully stressful experience.
“Working with Dennis again – and Stephen Daldry and James McAvoy both of whom I’ve admired for years was a joy.
“But it also felt like we were making something important.”