For actors it's slightly different, but the whole "becoming the character" part is, to me, the "writing like they speak" part. By fully inhabiting the character

For actors it's slightly different, but the whole "becoming the character" part is, to me, the "writing like they speak" part. By fully inhabiting the character, they will be compelled to speak in the manner written in the given situation. 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.