Most of the play's actions take place off stage.
- Nina's mention of Boris Aleskseyeevich to Arkadina reaveals that he is fishing, which is taking place off stage.
Arkadina: Prettily dressed and interesting looking.... That' agood girl... [kisses her] But we mustn't praise her too much or it may bring bad luck. Where's Boris Aleksyeevich?
Nina: He's down by the bathing-shed fishing.
- Medvidenkos mention of Sorin's habit of smoking
Medvidenko: Piotr Nikolayevich ought to give up smoking.
- Nina: Irena Nikolayevna's crying and Piotr Nikolayevich has got an attack of asthma.
- Most important of all, Trepliov's death too takes place off stage.
Dorn: [Turnin over the pages of a magazine, to Trigorin]
There was an article ............................... [Dropping his voice, in a lower tone] Take Irena Nikolayevna away from here somehow. The fact is, Konstantin Gavrilovich has shot himself...
Added to the above answer, there are some more incidents. Superlatively, Trepliov's first suicide attempt is not shown onstage, but only reported by a minor character, Masha. In act lll he's shown with a bandage on his head, but it is Masha who unwails the off-stage incident in the very beginning of act lll.
If he had wounded himself badly, I wouldn't have lived another minute. But I've got courage all the same.
There is an interval of two years within act lll and act iv. Act lll ends with Trigorin's and Nina's plan to meet in Moscow. Notably in act iv Nina's life has been totally changed. Now she has become a fallen woman by act iv and her crucial life incidents have been hidden among Chekhov's subtext. The revelation of these important incidents of her life is done by Trepliov.
She had a child. It died. Tigorin fell out of love with her and went back to his former attachments, as might have been expected. In fact he never gave them up, but in his spineless way he somehow managed to keep them all going. As far as I can make it out from what I've heard, Nina's personal life turnrd out a complete failure.
Masha's life too is revealed this way. She has a child but we are not shown her in pregnancy or anything regarding that. Only Medviedenko's request to her announces it to the audience.
Medviedenko: [Imploringly] Masha, do let us go! The baby may be hungry.
The fact that Trepliov has become a successful writer is also revealed by Trigorin. That process or him getting admirations is not made explicit onstage.
Your admirers send their greetings to you.... People in Petersburg and Moscow are vry intrigued by you always asking me what you are like, how old and whether you're fair or dark.
So, these incidents too happen off-stage.