Is Speaking in the Bathroom Haram?
Question

Is it permissible to speak in the bathroom or toilet? Is it necessary to close the door of the bathroom and turn on the light? How sound is that?
Praise be to Allah.Speaking in the bathroom
With regard to speaking in the bathroom, there are two scenarios.

The first scenario is: when a person is relieving himself. In this case, some of the scholars stated that it is disliked (makruh) to speak in all cases, whether one utters words of dhikr or anything else.

An-Nawawi (may Allah have mercy on him) said:
“It is disliked (makruh) to speak whilst relieving oneself, and the scholars are agreed on that.
Our companions said: All kinds of speech are equally disliked, but an exception is made in cases of necessity, such as if one sees a blind man falling into a well, or sees a snake, and so on, heading towards a human being or creature that is valued. In such circumstances it is not disliked (makruh) to speak; rather it is obligatory in most such cases.” (al-Majmu‘, 2/88)
The fuqaha of the four madhhabs stated that it is makruh to speak in this case. (See al-Mawsu‘ah al-Fiqhiyyah, 10/34)

The second scenario is if the individual is not relieving himself and is not uncovering his ‘awrah. If he says anything that involves mentioning Allah, may He be exalted, then many of the scholars have stated that it is disliked to do so inside the bathroom, by way of venerating the remembrance of Allah, may He be exalted, and avoiding uttering His name in dirty and filthy places.

As for any other kind of speech, the basic principle is that it is permissible, and we do not know of any textual evidence to suggest that it is not allowed.
Should the bathroom door be closed?
With regard to closing the bathroom door, if that is for the purpose of concealing the ‘awrah and as a precaution in case anyone walks past the bathroom door and sees the ‘awrah of the one who is inside, then it is obligatory, as is quite obvious, because concealing the ‘awrah is obligatory, and whatever is essential to fulfilling an obligation is also obligatory.
But if the person is alone in his house, and there is no one else with him, or the place where he relieves himself is in the bathroom, where it is not possible for anyone to see his ‘awrah, even if he walks past the bathroom, and other factors which mean that he is safe from anyone else seeing his ‘awrah, then in this case it is not obligatory to close the door of the bathroom, when he enters the room.
However, he should not fail to do that in all cases, even if he is certain that no one else will see his ‘awrah, so as to protect himself and to make sure that no one will hear whatever sounds come out of him, and the like, because undoubtedly feeling at ease, by closing the door, is more appropriate in all cases and is better in terms of etiquette.
It was the etiquette of the Prophet (blessings and peace of Allah be upon him) to seek out a place to relieve himself that was far away from people.
Imam at-Tirmidhi included a chapter in his Sunan to which he gave the title: “Chapter on what was narrated about how the Prophet (blessings and peace of Allah be upon him), when he wanted to relieve himself, would go far away.”
Then he narrated in that chapter (20) that al-Mughirah ibn Shu‘bah said: “I was with the Prophet (blessings and peace of Allah be upon him) on a journey, and the Prophet (blessings and peace of Allah be upon him) wanted to relieve himself, so he went far away.
At-Tirmidhi said: This is a hasan sahih hadith.” Classed as sahih by al-Albani.
As for closing the door for fear of the shayatin (devils), we do not know of any basis for that, and we do not know of any etiquette that specifically has to do with shayatin in toilets and places where people relieve themselves, apart from remembering Allah when entering the bathroom and seeking refuge with Him from the male and female devils.
Zayd ibn Arqam narrated that the Messenger of Allah (blessings and peace of Allah be upon him) said: “These toilets are inhabited by devils, so when any one of you goes to the toilet let him say, ‘A‘udhu Billahi min al-khubthi wa’l-khabaith (I seek refuge with Allah from the male and female devils).” Narrated by Abu Dawud (6); classed as sahih by al-Albani in Sahih Sunan Abu Dawud.
Al-Khattabi (may Allah have mercy on him) said:
“The word “hushush” (translated here as toilets) refers to places where people relieve themselves.
What is meant by “inhabited by devils” is that the shayatin frequent them.” (Ma‘alim as-Sunan, 1/10)
Ibn Daqiq al-’Eid (may Allah have mercy on him) said:
“These toilets are inhabited by devils ” that is, by the jinn and shayatin (devils). This highlights how appropriate this particular supplication is for this particular place.” (Ihkam al-Ahkam, 1/50)
Bathroom lighting
If what is meant by turning on the light in the bathroom when entering is that it plays a role in protecting a person from the harm of the jinn and shayatin, we know of no basis for that in the Quran or Sunnah, or in the words of the scholars. Rather, if there is no one inside the bathroom, then turning on the light is closer to extravagance and wasting money unnecessarily, and has nothing to do with the etiquette encouraged by Islamic teachings.
For more on bathroom-related fiqhi issues, please see these answers: 161383 , 20620 , 21195 and  2255 .
And Allah knows best.