We are given physical descriptions of the characters in the opening stage directions which also give us insight in to their motivations and attitudes.
All of the characters are in evening dress, suggesting that they abide by the traditions of their age and times past. Arthur Birling is identified quickly as middle-class as he is-
...rather provincial in his speech.
Mrs Birling is described from the outset as -
a rather cold woman
Which is later qualified in her attitude to the treatment of Eva Smith, by herself and the rest of the party.
Sheila is very happy at the beginning of the play: the celebration is the announcement of her engagement. She seems to be in anticipation, although the events that she is expecting do not play out as she may have expected.
Gerald is described as-
... very much the easy well-bred young man-about-town.
When we learn of his affair with Eva Smth/Daisy Renton whilst in a relationship with Sheila, we understand Priestley's description better.
Lastly we see Eric-
in his early twenties, not quite at ease, half-shy, half-assertive.
He is ashamed of his past and present behaviour, and is determined to atone for and accept his past indiscretions-
ERIC:...You lot may be letting yourselves out nicely, but I can't.