On the press night of Cinderella at the Lyric Hammersmith a few weeks back, whenever Julie Atherton's Cinderella and Mel Giedroyc's evil stepmother got within a few paces of each other, they were in danger of cracking up. Did it matter? Not a jot – corpsing, as it's called (possibly derived from an overwhelming desire to laugh when playing dead on stage) actually added to the enjoyment of the evening, rather than detracting from it.



Now, you probably don't want to see Hamlet breaking down in laughter during "To be or not to be", or Romeo and Juliet giggling during their death scene (although I've seen several on-stage deaths that have made me want to laugh uncontrollably). Pantomime is one of the few forms in which corpsing – and its close cousin ad-libbing – are often part of the fun, a reminder that what you are seeing is live and unpredictable. At the movies or TV, with performances fixed forever, you only ever see the corpsing in the out-takes at the end.

Some actors – Laurence Olivier was one; Tamsin Greig is another – are particularly prone to the habit. Like forgetting lines, sneezing and yawning, it can be a sign of a lack of concentration, or a production and a performance which has become tired and mechanical; in fact, during long runs, actors sometimes actually try to make each other laugh by playing on-stage practical jokes. Final performances have a tradition of such pranks. The danger is that while the actors are enjoying themselves the audience are often less entertained.

But the less a production tries to hide behind a fourth wall and the more it reaches out to an audience, the more enjoyable corpsing can be. What might be considered unprofessional in a traditional theatre production can feel joyous and very funny in less po-faced forms. Some theatres – Shakespeare's Globe in London is an obvious example – particularly lend themselves to corpsing because of the design of the auditorium, the playing style and the dynamic of the space. Whenever I've seen it happen there, it's always felt perfectly natural, and hasn't detracted from the production in the least.

Perhaps it is simply that in panto or spaces such as the Globe, where the audience presence is genuinely felt by the actors, those moments when they accidentally break character remind us all that the absurd but moving act of make-believe is a collaborative effort – unpredictable, delicate, messy and very human.