Blood loss from the scourging helped determine the time the victim survived. In any case, victims suffered a long time (at most, days) before falling into prolonged unconsciousness and death. Soldiers typically did not hasten things along because a long and painful death was the point of the execution method. Usually the victim was left on the cross until birds and wild beasts consumed the body.
Death could result from a variety of causes, including blood loss and hypovolemic shock, or infection and sepsis, caused by the scourging that preceded the crucifixion or by the nailing itself, and eventual dehydration. A theory attributed to French surgeon Dr. Pierre Barbet (author of A Doctor at Calvary: The Passion of Our Lord Jesus Christ As Described by a Surgeon) holds that, when the whole body weight was supported by the stretched arms, the typical cause of death was asphyxiation. He conjectured that the condemned would have severe difficulty inhaling, due to hyper-expansion of the chest muscles and lungs.
The condemned would therefore have to draw himself up by his arms, leading to exhaustion, or have his feet supported by tying or by a wood block. Indeed, the executioners were sometimes asked that the legs of the victim were broken or shattered, an act called crucifragium which was also frequently applied without crucifixion to slaves.
This act speeded up the death of the person but was also meant to deter those who observed the crucifixion from committing offenses. Once deprived of support and unable to lift himself, the victim would die within a few minutes.
Experiments by Dr. Frederick Zugibe, former chief medical examiner of Rockland County, New York have revealed that, when suspended with arms at 60° to 70° from the vertical, test subjects had no difficulty breathing, only rapidly-increasing discomfort and pain. This would correspond to the Roman use of crucifixion as a prolonged, agonizing, humiliating death. Zugibe claims that the breaking of the crucified condemned's legs to hasten death was administered as a coup de grâce, causing severe traumatic shock or hastening death by fat embolism. Crucifixion on a single pole with no transom, with hands affixed over one's head, would precipitate rapid asphyxiation if no block was provided to stand on, or once the legs were broken.
It is possible to survive crucifixion, if not prolonged, and there are records of people who did. The historian Josephus, a Judean who defected to the Roman side during the Jewish uprising of 66-72 AD, describes finding two of his friends crucified. He begged for and was granted their reprieve; one died, while the other recovered. Josephus gives no details of the method or duration of their crucifixion before their reprieve.
E. WERE VICTIMS CRUCIFIED NAKED?
It is still a matter of debate whether victims were crucified in the nude or with their loincloths left on. There is no doubt that many (if not most) crucifixion victims were stripped naked, either with or without a loincloth, as it would have humiliated the victim further. This is one of the elements which made crucifixion notorious: due to the physical, mental and emotional pain it caused.
While traditionally Jesus and the two criminals are depicted as having a sort of loincloth for modesty (in a few depictions, Jesus even wears a full-length robe, called a colobium), a few very early depictions depict the victim as either being stark naked on the cross or with some loincloth on (also see illustration at left and below right, one of which is a graffito found in Puzzuoli, with the other being a gem found in Syria, dating from the late 2nd-3rd century). As a general rule of thumb, most of these early representations are not depictions made by Christians, who still didn't depict the Crucifixion overtly during this time period, but were usually created by non-Christians and/or Gnostics.
While some take the position that Jesus was not spared even a loincloth when He was crucified, some believe that due to Jewish sensibilities, loincloths were left on or provided (it would be fitting to remind here that many people in ancient times did not even wear loincloths; for them, their tunics served as their undergarment). So, before we could have any conclusive evidence, it would seem that the best answer here for the moment is that it depended on the situation and the location.
F. THE SHAPE OF THE CROSS
The gibbet on which crucifixion was carried out could be of many shapes. Josephus records multiple tortures and positions of crucifixion during the Siege of Jerusalem as Titus crucified the rebels; and the Roman historian Seneca the Younger recounts ("To Marcia, On Consolation", 6.20.3):
"Video istic cruces non unius quidem generis sed aliter ab aliis fabricatas: capite quidam conversos in terram suspendere, alii per obscena stipitem egerunt, alii brachia patibulo explicuerunt. Video fidiculas, video verbera, et membris singulis articulis singula docuerunt machinamenta: sed video et mortem..."At times the gibbet was only one vertical stake, called in Latin crux simplex or palus. This was the simplest available construction for torturing and killing the criminals. Frequently, however, there was a cross-piece attached either at the top to give the shape of a T (crux commissa) or just below the top, as in the form most familiar in Christian symbolism (crux immissa). Other forms were in the shape of the letters X and Y.
I see there crosses, not merely of one kind but fashioned differently by others: a certain one suspends with head down towards the ground, others drive stakes through their private parts; others stretch the arms out on the gibbet; I see cords, I see whips, and contraptions designed to torture every joint and limb, but I see death as well...
While the view that Jesus died on a stake has thus been propounded by writers of the nineteenth and twentieth century (and is still popular among Jehovah's Witnesses), second-century writers, such as Justin Martyr and Irenaeus, who were much closer to the event, speak of him only as dying on a two-beam cross.
In the same century, the author of the Epistle of Barnabas and Clement of Alexandria saw a two-beam shape of the cross of Jesus as foreshadowed in a numerological interpretation of Genesis 4:14, and the first of these, as well as Justin Martyr, saw the same shape prefigured in Moses keeping his arms stretched out in prayer in the battle against Amalek. At the end of the same century, Tertullian speaks of Christians as accustomed to mark themselves repeatedly with the sign of the cross, and the phrase "the Lord's sign" (τὸ κυριακὸν σημεῖον, to Kyriakon simeion) was used with reference to a cross composed of an upright and a crossbeam. Crosses of † or Τ shape were in use, even in Palestine, at the time of Jesus.
See here for more in-depth discussion on the shape of Jesus' cross.
G. LOCATION OF THE NAILS
In popular depictions of crucifixion, the condemned is shown with nails in the palm of their hands. Although historical documents refer to the nails being in the hands, the word usually translated as hand, "χείρ" (cheir) in Greek, referred to arm and hand together, so that, words are added to denote the hand as distinct from the arm, as "ἄκρην οὔτασε χεῖρα" (Akrin outase cheira, "he wounded the end of the 'cheir'", i.e. he wounded her hand).
A possibility that does not require tying is that the nails were inserted just above the wrist, between the two bones of the forearm (the radius and the ulna). The nails could also be driven through the wrist, in a space between four carpal bones. The word χείρ, translated as "hand", can include everything below the mid-forearm: Acts 12:7 uses this word to report chains falling off from Peter's 'hands', although the chains would be around what we would call wrists. This shows that the semantic range of χείρ is wider than the English hand, and can be used of nails through the wrist.
An experiment that was the subject of National Geographic Channel's documentary entitled Quest For Truth: The Crucifixion, and of a brief news article, showed that a person can be suspended by the palm of their hand. Nailing the feet (or the ankles) to the side of the cross relieves strain on the wrists by placing most of the weight on the lower body.
Another possibility, suggested by Frederick Zugibe, is that the nails may have been driven in at an angle, entering in the palm in the crease that delineates the bulky region at the base of the thumb, and exiting in the wrist, passing through the carpal tunnel.
A footrest attached to the cross, perhaps for the purpose of taking the man's weight off the wrists, is sometimes included in representations of the crucifixion of Jesus, but is not mentioned in ancient sources. These, however, do mention the sedile (a small piece or block of wood attached to the front of the cross, about halfway down, where the victim could rest) which could have served that purpose.
H. THE NUMBER OF THE NAILS USED IN JESUS' CRUCIFIXION
The question has long been debated whether Jesus was crucified with three or with four nails.
The treatment of the Crucifixion in art during the earlier Middle Ages strongly supports the tradition of four nails, and the language of certain historical writers (none, however, earlier than Gregory of Tours, "De Gloria Martyrum", vi), favors the same view. The earliest depictions of the subject (see links to pictures in Section E above) might also favor this view, as they generally depict the feet of the victim as being separate from each other.
On the other hand, in the thirteenth century, most of Western art (with a few exceptions; see the image to the right, painted by Diego Velázquez in 1632) began to represent the feet of Jesus as placed one over the other and pierced with a single nail. This accords with the language of Nonnus and Socrates and with the poem "Christus Patiens" attributed to St. Gregory Nazianzus, which speaks of three nails.
This depiction of three nails had actually caused some controversy when it was first introduced. For example, in the latter part of the 13th century the bishop of Tuy in Iberia wrote in horror about the 'heretics' who carve 'ill-shapen' images of the crucified Jesus 'with one foot laid over the other, so that both are pierced by a single nail, thus striving to annul or render doubtful men's faith in the Holy Cross and the traditions of the sainted Fathers.'
Archaeological criticism has pointed out however not only that two of the earliest representations of the Crucifixion (the Palatine graffito does not here come into account), viz., the carved door of the Santa Sabina in Rome, and the ivory panel of the British Museum, show no signs of nails in the feet, but that St. Ambrose ("De obitu Theodosii" in P.L., XVI, 1402) and other early writers distinctly imply that there were only two nails. However, this does not answer why in Luke 24:39-40 Jesus is said to have shown 'his hands and his feet' to his disciples, unless there was some distinguishing mark located there.
St. Ambrose informs us that Empress Helena had one nail converted into a bridle for Constantine's horse (early commentators quote Zechariah 14:20, in this connection), and that an imperial diadem was made out of the other nail. Gregory of Tours speaks of a nail being thrown (deponi), or possibly dipped into the Adriatic Sea to calm a storm. It is impossible to discuss these problems adequately in brief space, but the information derivable from the general archaeology of the punishment of crucifixion as known to the Romans does not in any way contradict the early Christian tradition of four nails.