This is one example of an improvised moment on the big screen. It is from a wonderful film called Midnight Cowboy. This film had fantastic music, a great story but most importantly great acting. If you think of this film this is one of the moments you will properly remember and you would never have thought that it was improvised.
Another one of my favourite scenes from a film is a scene from On The Waterfront. Marlin Brando is walking through the park with Eva Marie Saint and she drops her glove by mistake. So naturally he picks it up but he decides to play with it and put it on his hand. This scene would have been completely different if he had just left it. By putting it on and playing with it in a way changes the point of the scene. It enhances the connection between the two actors and shows that he likes her. You would think that this would have been the direction the director had given.
The last scene I want to talk about is from Dr Zhivago. Hans Matheson has just come back from the war and Keira Knightley is bathing him. In the script she was just meant to bath him but she decides on the spot to climb into the bath. This is just another great example of how actors have the drive to do what is right for their character and the film. I could not find this clip on youtube but I have found a clip of Hans Matheson talking about the scene.
It is important to have great acting in a film and it is what separates the good films from the fantastic films. However you do need the good actor but you need a good character for that actor to play. If you think to Righteous Kill with Robert De Niro and Al Pacino it is pretty awful. You can not say that they are bad actors but you can say that their performances where not good. The reason there performances are not good is down to them not being able to challenges themselves within a role.
So to make your film fantastic you do need your great actors but they are just tools you need to let them work by giving them a part they can lose themselves in and then you will end up with wonderfully improvised scenes and powerful performances.