There's no denying that the two have very different strengths and weaknesses - one of the recurring conversations at ScriptTank, my writers' group, relates to the need for a story to find the medium to which it's best suited, be that theatre, film, television, radio or prose. So, in the interests of sparking debate, I'd like to propose a formulation. The difference between the two major dramatic media is, at root, elemental: film is fire and water, theatre is earth and air.
Still reading? Let me elaborate...
Film is defined by a changing pattern of light projected onto a wall. Once upon a time, it used to flicker, conveying a sense of stories by the fireside, but digital projection has lessened that, more's the pity. On this light, we witness a constantly shifting, fluid image, the human brain finding causality and connection between discrete shots. Film-makers have picked up on these elements since the medium's birth. The Life of an American Fireman (1903) was one of the first narrative films, and fire, in the form of explosions, is still the defining feature of the summer blockbuster. Major movies have been set on ships from Potemkin to Titanic. The most iconic film of them all starts with liquid in a snow-globe, and ends with a sledge being consigned to a furnace.
The theatre, by contrast, is defined by shared space and physical reality. Whatever you see on stage is there, in real time and one take. It's why, for instance, the steam train in The Railway Children is far more impressive than its cinematic equivalent - it's actually there, not just 3-D but in Einsteinian terms, 4. Added to that, there's the knowledge that everyone present is sharing the same experience, breathing the same air. One of its greatest pleasures is that moment of realising that everyone in the room is holding their breath at the same time, producing what Shelley Winters described as 'the best sound a player can get... the sound of a wonderful, deep silence that means you've hit them where they live'. (Indeed, my own job - directing - can be described as the art of getting an audience to synchronise their breathing - it's harder than it sounds).
So there you go - light and fluidity equals fire and water, solidity and sharing equals earth and air. Two media, each with their own defining elements. Logically, you'd have thought that both would be at their best when playing to their strengths, and that's often the case. But sometimes the greatest joy comes from work that plays against its medium, and triumphs. Many of my own best experiences in the theatre have had a fluidity of image that I only call 'cinematic' - funnily enough, these tend to be either extremely high-tech - the work, for instance, of Robert Lepage - or very low-tech, like Trevor Nunn's bare-stage Macbeth.
Conversely, a film like Hunger (2008), about the IRA hunger striker Bobby Sands, directed by the artist Steve McQueen and written by the playwright Enda Walsh, uses its long takes and loving, very corporeal close-ups (often of things that aren't normally considered beautiful - starvation-racked bodies and shit-stained walls) to create an experience that was much closer to a live event. Looking round me in the cinema, I realised that I was hearing that silence described above - a silence that is, in the best sense of the word, theatrical.