Verbalizing the words silently: Those that are reading along in a Kosher Megillah together with the Baal Korei are to verbalize the words silently.
Are those that read along to themselves in a Kosher Megillah considered to fulfill the Mitzvah with a Minyan?
Yes. Some rule this applies even if he does not read word for word with the Chazan. Others rule that if he does not read word for word with the Chazan then it is not considered that he is reading with a Minyan and hence on Purim Meshulash one is to avoid reading with the Chazan even in a Kosher Megillah.
B. A Chumash or invalid Megillah:
One who does not have a Kosher Megillah available is to read along in a Chumash or printed Megillah.
Not to verbalize the words: One who is following along in a non-Kosher Megillah or in a Chumash is not to verbalize the words and is rather to follow the reading silently in his thoughts. Some Poskim rule this applies even if the person with the Chumash is the only listener to the reading. Others however rule that in such a case, that he is the only listener to the reading, then he may read quietly to himself from the Chumash.
One who missed a word of the Chazan: If one did not hear a word from the Chazan he must verbally read that missed word. He must continue reading from the missed word until he catches up to the Chazan’s reading, in which case he is to stop verbalizing the words and listen to the reading.
When Haman’s name is recited: Due to the noise level made during the recitation of Haman each person is to read to himself from a Megillah or Chumash one or two verses in proximation to “Haman”, just in case the Chazan has already continued reading and one is unable to hear. [In many communities today the custom is for the Chazan to remain silent until all the noise in the crowd has stopped.]
 P”M 689 A”A 11; M”B 689/19; 690/26; Kaf Hachaim 689/27; 690/36
The advantage of a Kosher Megillah over a Chumash: Initially one is to hear all the words read from a Kosher Megillah. Hence it is proper that one have a Kosher Megillah to read along with the Chazan in case he misses a word. [Kaf Hachaim 690/36] Alternatively this is to suspect for those Poskim that rule that one who was thinking of other matters while hearing the words is considered as if he did not hear those words. Thus one is to verbally read along in a Kosher Megillah, as in such a case even if he does not pay attention to the words he still fulfills his obligation. [Nemukei Orach Chaim 690; See Halacha 15 in Q&A!]
 P”M 689 A”A 11; Kaf Hachaim 689/27
The reason: One is to verbalize the words with the Baal Korei, in order to guarantee that he does not miss even one word from the Megillah, as according to some Poskim if even one word is not heard, one has not fulfilled his obligation, as explained in Halacha 15.
 M”B 689/19; Shaar Hatziyon 690/11 based on reason of M”A 690/6 that the other listeners may focus on the words of this person, and this person reading, which is not the Chazan, does not have in mind to fulfill the obligation of others. [ibid]
 Piskeiy Teshuvos 690/10
 Chazon Ish 155/2
 Griz of Brisk
 M”B 690/19 in name of Peri Megadim 690 A”A 7; Kaf Hachaim 690/36
The reason: The reason for this is because it is difficult for listeners to hear all the words due to the noise made during Haman. Hence they should read in the missing words from the Chumash and at the very least fulfill their obligation Bedieved. [ibid]
 Michaber 690/4
The reason: The reason for this is because perhaps a listener will listen in to the words of the Chumash reader rather than the Chazan who is reading from a Kosher Megillah. However the reader himself will certainly pay attention to the Chazan and not his own reading. [M”A 690/6 based on Beis Yosef] Alternatively the reason is because the reader himself will pay attention to his own reading and ignore the reading of the Chazan from the Kosher Megillah. [Levush, brought in Kaf Hachaim 690/27; M”B 690/13]
 Mamar Mordechai 690/3; Elya Raba [brought in M”B 690/14]; M”B 690/13-14; Kaf Hachaim 690/27 based on second reason mentioned above that the Chumash reader will pay attention to his own reading and ignore the reading of the Chazan from the Kosher Megillah
 Machatzis Hashekel 690/6 based on M”A ibid which writes the first reason mentioned above. [Brought in M”B and Kaf Hachaim ibid]
 M”B 690/19; 690/60; Peri Megadim 690 A”A 7
 M”A 690/19; M”B 690/60; M”B 690/19 in name of Peri Megadim 690 A”A 7; Kaf Hachaim 690/36