Considering that actors are often the most famous names associated with a given project, they're right up there with the directors, producers and writers of a film in terms of how much pressure they have on their shoulders.
And just like everyone else involved in the creative process, the stars can bring their own ideas and quirks to the table, resulting in a lot of possible avenues down which to take each individual scene. But not all of these avenues are made of solid gold.
In fact, because of how unpredictable and difficult movie-making can be, some actors have even come close to ruining an important scene, whether by contributing an idea that sounds like it would've been terrible, spoiling a strong take with a poorly timed reaction, or simply by being reluctant (or unable) to perform an important moment.
Fortunately, these sequences turned out great in the end, but their magic could've been diminished - or even vanquished entirely - if the actors' odd behaviour, wild ideas or bad on-set conduct had been allowed to influence the final outcome of the scene.