The Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909), in Parashat Vayetze (Shana Rishona, 4), writes that one must always close the door when using the restroom, even in the dark of night. Despite the fact that nobody can see him, one must nevertheless ensure to close the door to the restroom in the interest of Tzeni'ut (modesty).
Later (Halacha 10), the Ben Ish Chai rules that it is forbidden to speak even a single word while in the restroom, given the restroom's status of Tum'a (impurity). He emphasizes that this pertains not only to words of Torah, but even to mundane matters. (Chacham Baruch Ben-Chayim told that once when he was in the Yeshiva's restroom another faculty member began speaking to him, but he did not respond. The other faculty member felt insulted until Chacham Baruch explained to him the Halacha forbidding speaking while using the restroom.)
The Ben Ish Chai further discusses (in Halacha 15) the importance of reciting the Beracha of "Asher Yatzar" after using the restroom. He notes that women, too, are required to recite this Beracha, and it is proper for them to commit the text to memory to help ensure that they can recite it each time they leave the restroom. Beyond the strict Halachic requirement, the Ben Ish Chai notes, the recitation of this Beracha also achieves significant "Tikunim" ("corrections" in the upper worlds) and it is therefore of utmost importance for both men and women to recite this Beracha.
Elsewhere (Od Yosef Chai, Parashat Vayetze, 2), the Ben Ish Chai discusses the Halacha requiring that one refrain from all other activity while reciting a Beracha. He bemoans the fact that so many people are negligent in this regard particularly when reciting the Beracha of "Asher Yatzar." People often recite the Beracha as they dry their hands or return to their normal activities. This is incorrect; while reciting this Beracha and any Beracha one must stand in place and refrain from all other activities, and concentrate on the Beracha he recites.
Summary: One who uses the restroom should close the door even if nobody is present and even in the dark of night. It is forbidden to speak in the restroom. Both men and women must ensure to recite "Asher Yatzar" upon leaving the restroom, and when reciting this Beracha and any Beracha one must not perform any other activity, including drying his hands or walking.
