All the chemicals used in America are standard medical drugs. Sodium thiopental is a short acting barbiturate which is used widely as an anaesthetic and normally causes unconsciousness very quickly if injected into a vein. Pavulon is a muscle relaxant that paralyses the diaphragm and thus arrests breathing whilst Potassium chloride finishes the job by causing cardiac arrest. It is used in cardiac surgery to stop the heart.
In most cases the prisoner is unconscious about a minute after the Sodium thiopental has been injected and is dead in around eight minutes, with no obvious signs of physical suffering.

In some states a fully automated lethal injection machine is used that runs off a 12 volt battery. It injects the chemicals in the right order and amount once the catheters are in place. This considerably assists matters and avoids the problems of mixing of the chemicals which can lead to a reaction causing precipitation which makes them impossible to inject.
The machine has eight syringes. Three with lethal Chemicals that are electrically operated, three with lethal chemicals which are mechanically operated (back-up) and two filled with saline solution used to bleed or purge air form the manifold and the IV lines. Two buttons control the machine, one for the lethal syringes and one for the identical looking harmless ones. The two executioners each press a button, and the syringes release the drugs into the IV line.

Multiple executions.
In Arkansas in 1994, prison officials, citing the disruptive impact of executions on staff and other prisoners and the high cost of "rehearsal time" and overtime pay, took steps to reduce both problems by conducting multiple executions. They carried out two unrelated executions on the night of May the 11th and a further three on August the 3rd of that year with a 45 minutes interval between them so that the chamber could be cleared between each prisoner and to allow prison officials time to replace needles and tubes used to administer the injection. The sheet on the gurney was also changed between executions.
Texas has also carried out multiple executions. On 30th January 1995, in that state's first multiple execution in 44 years. 33-year-old Clifton Russell was put to death just after midnight and Willie Williams, 38, was injected about an hour and a half later. Texas carried out a further double injection in 1997. (Dorsie Johnson-Bey & Davis Losada on the 5th June)

Is lethal injection the humane alternative?
Execution by lethal injection takes much longer from start to finish than any other method, typically 30 - 45 minutes depending on the execution protocol and ease or otherwise of locating a vein. In the U.K. a hanging took around 15 seconds to carry out in the later part of the 20th century. For the majority of this time the condemned person is fully aware of what is happening to them and able to experience their execution. They know that they will be dead at the end of it and the fear of suffering (particularly in front of an audience) and of the unknown is strong in most of us. It is difficult to see therefore how it can be considered more humane, as the prisoner is subjected to far more mental anguish over a longer period.

It is fair to say that injection is much less dramatic than the electric chair or hanging and probably easier for the staff and witnesses as it looks more like a surgical procedure than an execution. But does it cause the prisoner less suffering overall?
When all goes well, the only physical pain is the insertion of the catheters. If the person's veins are easy to find this can be done in a minute or so. The catheters are connected to the saline drip and the prisoner is wheeled into the execution chamber where they are in full view of the witnesses and journalists. After they have made their final statement the injection of the lethal chemicals can begin and they may pass almost instantly from full consciousness into unconsciousness or they may feel themselves becoming drowsy and know that they are beginning to die.

In modern hanging they are alive one second and unconscious the next (if everything goes to plan). It is unlikely that they feel themselves slipping into death.
Not everyone is of the opinion that death by lethal injection is painless - Dr. Edward Brunner, chairman of the Department of Anaesthesia at North-western University Medical School, submitted an affidavit on behalf of death row inmates in Illinois in which he states that lethal injection "create[s] the substantial risk that prisoners will suffocate or suffer excruciating pain during the three chemical injections but will be prevented by the paralytic agent from communicating their distress." It is notable also that Albert Pierrepoint, who was one of Britain's most prolific hangmen and who witnessed an early lethal injection execution considered that the process was "sadistic" mainly due to the length of time it took to render the prisoner unconscious.
In the minds of the American public and of jurors in capital cases the perception of lethal injection is of a clean, clinical and painless end. 71% of those responding to my 2001 survey considered injection to be the least cruel form of execution.

This perception is a great advantage to the state because the public are much more willing to accept execution in this form and jurors more willing to convict and pass the death sentence. The media interest in the eventual execution is also diminished. Texas which has carried out around a third of all post 1977 executions finds that there is now very little interest in them unless the criminal is particularly notorious and thus avoids much of the protest that attends other methods.
There was intense media interest in the three hangings in Washington and Delaware and the two shootings in Utah because these methods were perceived as old fashioned and barbaric.

Utah may well abandon shooting in favour of injection not because either execution was in any way botched but because they were perceived as being gruesome. It is probable that there will be only a very few if any more hangings in Washington for the same reason.
One wonders, however, if injection is as much of a deterrent as hanging or the electric chair, to would be criminals or whether they feel it is a "soft option"? This is an important point because if the state is going to take the life of a person surely it should seek to produce the maximum deterrence from doing so within the realms of a providing the prisoner with a quick and pain free death. Interestingly in those states that allow the prisoner the choice of execution method, not everyone does choose injection.