"There are a lot of visual marks that have to be hit, and lines that need to be said in a right way - so there wasn't really any improvisation on the set when it came to the bulk of the script"

Simon Pegg's quote is describing the value of following the script when shooting a motion picture. He is stressing that there are certain visual components and lines that need to be said in a particular way, so there is no space for improvisation. This indicates that the actors must stay with the script and not deviate from it. This is necessary for the director to make sure that the movie corresponds which the story is told in the way that they planned. It also makes sure that the movie is cohesive which the audience can follow the plot. Pegg's quote is a suggestion of the value of following the script and not differing it. It is a suggestion that the director has a vision for the motion picture which the actors need to respect that vision and adhere to the script.