If you watch any side by side of a film audio / final and the script you'll realise what you're saying simply isnt true.
It's very rare that a conversation scene mirrors the dialogue exactly 1 to 1. Obviously there will be certain lines where the director wants exact delivery but actors very often deliver a slightly different line than as written.
If anything this frequency increases the higher level of profile / skill the actor has.
There are countless examples where a director is asked about scenes and defers the credit to the actors for improvising something particularly well or coming up with a better line to convey the same point - they are the ones, after all, in charge of personifying the character that was written. They may feel a different delivery suits the character better.
It's very rare that a conversation scene mirrors the dialogue exactly 1 to 1. Obviously there will be certain lines where the director wants exact delivery but actors very often deliver a slightly different line than as written.
If anything this frequency increases the higher level of profile / skill the actor has.
There are countless examples where a director is asked about scenes and defers the credit to the actors for improvising something particularly well or coming up with a better line to convey the same point - they are the ones, after all, in charge of personifying the character that was written. They may feel a different delivery suits the character better.