Discuss in detail the idea that a dramatist not only lets the actors be creative while performing onstage but also must make sure that the larger thematic impact of the play should remain unchanged.

The term "dramatist" is a synonym for "playwright."
Playwrights vary in the degree to which they want to open up space for creativity in staging and performance. The degree to which this is done depends on the dramatic tradition. For example, producers of classical Greek drama worked within highly standardized traditions and thus did not innovate in staging plays. Shakespeare wrote his plays for his own company and thus would have control over how they were staged, although modern performers and directors have created many different types of interpretations. Shaw uses extensive stage directions to ensure that performers conform to his vision of how the plays were to be staged, whereas in earlier English plays, stage directions were much sparser.
Thus one can say that some playwrights create scripts that deliberately allow room for multiple interpretations in performance, while others use stage directions, prefaces, and narration to limit such flexibility.