Firstly: The Ruling of Standing in Prayer The ruling of standing during the wajib prayer
Standing during the wajib prayer is wajib whenever one is able to do so. Consensus on this has been quoted by Ibn `Abd al-Barr, Ibn Hazm, Ibn Rushd, al-Nawawi, and Ibn Nujaym. The ruling of standing during additional prayers
Standing during additional prayers is not wajib. It is permissible to offer a voluntary prayer while sitting even when able to stand. Consensus on this has been quoted by Ibn `Abd al-Barr, al-Nawawi, and Ibn Qudamah. The definition of standing
The definition of standing in prayer is that one stand straight and upright. Slight slouching is not problematic. This is by agreement of the four schools of jurisprudence.  The definition of standing according to Hanafis is that the knees cannot be touched with the hands outstretched. According to Shafi`is, slouching that negates standing is when one is closer to bowing. The definition of standing according to Hanbalis is to stand erect until one bows.
Secondly: Standing Unassisted and the Ruling of Leaning on Something
It is wajib for one able to stand unassisted during the wajib prayer to do so. It is not permissible to stand while leaning on something such that one would fall if it were removed. This is explicitly mentioned by Malikis and Hanbalis, and is a position among Shafi`is. It is the selected opinion according to Ibn `Uthaymin.
Thirdly: The Ruling of Standing for Prayer on a Boat
It is wajib to pray standing in a boat when possible. This is the position of the majority: the Malikis, Shafi`is, and Hanbalis. Among Hanafis, Abu Yusuf and Muhammad ibn al-Hasan were of this position. It is also the position of Ibn Hazm.
Fourthly: The Ruling of Standing for Prayer on an Airplane
The ruling of praying on an airplane is the same as the ruling of praying on a boat: it is wajib to pray standing when possible. Otherwise, the prayer is offered as best as one can. This is the position of Ibn Baz, al-Albani, and Ibn `Uthaymin.
Fifthly: Where One Praying Looks When Standing
The majority – Hanafis, Shafi`is, and Hanbalis – prefer that the one praying look at one’s place of prostration while standing.