When we watch movies, we tend to assume that pretty much everything we see on the screen has been carefully planned - movies, after all, cost a lot of money, and with studio execs breathing down the necks of the production team there's unlikely to be much room for improvisation and experimentation. Of course, there are well known exceptions - Mike Leigh has a strong reputation for allowing his actors plenty of room to breathe and develop their characters intuitively, and Richard Linklater has a similar disposition with his cast. But even Leigh and Linklater lock things down to a degree once the cameras start rolling, and their projects are often relatively low key, character-driven pieces. The idea that bigger productions could do the same seems almost counter-intuitive to our idea of film production - we imagine a meticulous attention to detail in which there is no room for error carried out with the precision of a military operation. But you'd be surprised just how many classic movie moments occur off the cuff, where actors fill in the gaps and improvise, often with some amazing results. Here are ten cases of on-set improvisation which resulted in iconic movie moments - see the clips for yourself and be prepared to be impressed.