The entire piece is improvised. Mike Leigh worked with each actor separately to create a character with a history from birth to the present day. Once the characters were fully formed he placed them in the situation and let the action commence. Probably about 90% of each character was left unseen, which is why it appears that Laurence, Sue and Tony are preoccupied with things, but we never discover what they truly are - which adds to the tension.
During the film, Angela mentions that Tony used to play, in the first team, for Crystal Palace when he was in his late teens. John Salthouse, who plays Tony, played for the same team, around the same time as Tony did.
The use of the song by Demis Roussos in the teleplay which became so synonymous with Abigail's Party, actually replaced the one by José Feliciano which was used in the stage production, because they couldn't get permission to use Feliciano's name or music.
The play got a huge audience on the Sunday night it was screened in 1977. In the UK in those days there were only three TV channels: one (ITV) was on strike and the other (BBC2) was affected by a large storm causing all viewers to tune into the BBC station (BBC1) screening the play.
Alison Steadman based Beverly on a lady she knew whilst at Drama school in Essex, she merged this person with a woman she saw demonstrating a make-up range at a department store who either knowingly or unknowingly humiliated a lady she had plucked from the passing shoppers and telling a watching crowd she had applied her lipstick very badly.
When Beverly puts on a record of "Love to Love You, Baby", it is not Donna Summer singing. Her version of the song had been banned by the BBC because it featured "orgasmic moans". Instead the play used a cover version by Clare Torry.
The cramp Angela is crippled by at the end of the play was originally genuine. During rehearsals for the stage play Janine Duvitski was suddenly struck down with cramp and doubled over, Mike Leigh ruthlessly cried "Keep Acting" and the whole thing ended up remaining in the stage and TV play.