This doesn’t make much sense: actors don’t write their material, they are given it. They can’t write like they speak, and also they can’t improvise
And as the other comments mention, actors very often do alter the lines. Behind the scenes footage and interviews with actors reveal this often happens because they think the screenwriter/director got it wrong in that particular moment -- that their character wouldn't respond like that.
The Han Solo example is a good one. Hours of takes saying "I love you too" and then Harrison Ford has a flash of clarity and realizes Han would NEVER respond like that. Calls for a quick take, "becomes Han Solo", says "I know" and the rest is history.
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 rare they say all lines exactly as in the script. In fact often the script gets updated with ideas that came up during shooting including improvised lines.
The biggest problem are writer/directors who cannot deal with improvising actors. Like a Haneke, who would turn around in his grave if an actor goes against his sacred script. That's why they have 25 shots per scene, and 3-6 months per shoot.
Sometimes a line just isn't working and an actor or the director or the writer or a grip will come up with something that works and that's what you hear about. Those are exceptions and not the rule.
The director has the final say. Often others higher up have the final say. If he wants you to say the line as written, you will say the line as written.
In general it's more common that some lines will change and be improvised by the actors, than not.
It's even practical, some lines come off as stiff when the actors tell them verbatim, others just can't be replicated in a longer emotionally charged scene (where the flow and the emotions carry the performance), and so on.
>Often others higher up have the final say. If he wants you to say the line as written, you will say the line as written.
DUH!