To be able to adjust to whatever the gods have in store for us when we begin shooting, and allow all two hundred of us to react effectively, a good director has a clear plan at the outset. Ang Lee's vision of Brokeback Mountain was very clear.
The script, of course, is the blueprint for the film. The clearer the blueprint the better foundation you have for making a good film. Since we had the best script most of us had ever read we had a great advantage from the beginning. However, it would be almost impossible to visualize an entire film in advance without making adjustments. In fact, keeping too rigidly to a plan can keep a director from
xploiting opportunities that inevitably arise, as well.
Ang Lee had worked very hard to communicate his vision of each scene to us as far as he could. We were given his breakdown of what he needed for each scene and we provided the elements. How closely the finished film resembles what he imagined in pre-production, again, can only be answered by Mr. Lee.
As I said, we were given a breakdown of what Mr. Lee needed for each scene. For example, he may have requested wind and dust, which I then passed on to the special effects team. Or he may (in a perfect world) want a cloudy sky, which might require sky replacement if it's sunny on the day we were to shoot that scene, which means I have to pass this along to the visual effects team and ensure that our second unit shoots plate shots of cloudy skies.

Cloudy skies over Signal (Cowley, AB.)
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Pierre Tremblay Interview