The reader: The Megillah may be read in either a sitting or standing position. However by a public reading the reader must stand out of honor for the congregation. [Nevertheless in a time of need it is permitted for him to lean on an item, even in a way that the item will support him from falling. By a private reading the reader may even initially read the Megillah while sitting.]
The listener: The listeners are not required to stand during the Megillah reading. [Nevertheless some are accustomed to stand during the reading.]
The blessings: The blessings must be recited in a standing position. This applies in all cases, whether one is reading for a Minyan or in private. [This applies for both the before and after blessings.] Even the listeners are to stand at the time of the recital of the blessing. However some rule that it is not necessary for the listeners to stand for the recital of the blessing and so is the custom of certain communities.
The blessings are to be recited standing. Some are accustomed that the entire congregation stands during the blessings. By a public reading the reader is to stand during the reading, while the congregation may sit. By a private reading even the reader may sit during the reading, after the blessings is recited.
If one is reading together with the Baal Korei from a Kosher Megillah is he required to stand?
No. Nevertheless some are accustomed to stand, as stated above.
 Michaber ibid; If however the congregation agrees to allow the reader to sit, he may do so. This is in contrast to Kerias Hatorah in which case allowance of the congregation is invalid. [Mateh Yehuda brought in Kaf Hachaim 690/5]
 Biur Halacha 690 “Aval Lo Yikra” based on Mateh Yehuda who rules leaning is permitted; Kaf Hachaim 690/4; However see 585/1 and 607/7 that leaning in a way that the person would fall without the support is considered like sitting. Thus the M”B ibid only allows it in a case of need.
 Vetzaruch Iyun as to the definition of a private reading or a “congregation”. Seemingly a congregation refers to one who is reading in front of a Minyan of people while if he is reading to a few individuals it is considered a private reading and he may sit. It certainly is not considered a public reading if he is reading for a single individual. Vetzaruch Iyun.
 This is even initially permitted. [Maggid Mishneh; Ran; Bach; Kaf Hachaim 690/1]
 The Rebbe and Rebbe Rayatz would stand while hearing the reading. [Otzer Minhagei Chabad 83-84]
 M”A 690/1; Elya Raba 690/1; P”M 690 A”A 1; Beis Oved 690/9; Rameh 102; M”B 690/1; See Admur 8/3 that all Birchas Hamitzvos must be said standing. See regarding the blessing over Shofar: Admur 585/4; M”E 585/3; M”B 585/1; Kaf Hachaim 585/1
 P”M 690 A”A 1; Beis Oved 690/9; Ben Ish Chaiy Tetzaveh 4 See Admur 8/3 that all Birchas Hamitzvos must be said standing. There is no difference whether one is fulfilling the Mitzvah privately or for the public.
Other Opinions: Some argue that it is not necessary for a private reader to stand while reciting the blessing and that their custom is not to stand. [Kneses Hagedola 392; Tefila Ledavid 86; Orah Vesimcha 14a; Yifei Laleiv 2/2; Kaf Hachaim 690/2-3]
 Ben Ish Chaiy Tetzaveh 4; Piskeiy Teshuvos 690/1; However see Sheivet Hakehasi 1/212
 Machatzis Hashekel on M”A 690/1; Beis Oved 690/9 brought in Kaf Hachaim 690/2; Shaar Hatziyon 690/1; Vetzaruch Iyun as to the opinion of Admur in all this, as in 585/2 Admur rules that the people fulfilling the Mitzvah of Tekia do not have to stand, despite the fact that the person blowing the Shofar has to stand. No mention is made that they have to stand while reciting the blessing. Vetzaruch Iyun.
The reason: As the listener is considered like the reader, “Shomeia Keoneh”, and hence is required to stand just like the reader which is saying the blessing. [ibid]
 Yifei Laleiv 2/2; Kaf Hachaim 690/2 that so was custom of the Beis Keil Shul in Jerusalem; Mikraeiy Kodesh brought in Piskeiy Teshuvos 690/1; See Ben Ish Chaiy Netzavim 15 brought in Kaf Hachaim 585/1 regarding the blessing of Shofar that in Bagdad the custom is not to stand.
 The reason: As this silent reading is no different than a private reading which may done sitting.