The 'Answer to the Ultimate Question of Life, the Universe and Everything' does pop up every now and then throughout the books within the Hitchhiker series. However, the real meaning has always remained elusive at best. With the chance that the other books in the series will be turned into movies being very small, here below is a summary for those who are interested to know:
It was revealed at the end of the first book that Earth and all its living beings were originally constructed 10 million years before, as a living supercomputer designed to calculate The Ultimate Question itself. However, Earth was destroyed by the Vogons five minutes before the Question was to be produced. In the second book, The Restaurant at the End of the Universe, Ford Prefect reasons that Arthur Dent, being an Earthling, still carries part of the Earth's original computing matrix within his subconscious brainwave pattern. So Arthur randomly picks Scrabble letters from a bag, which form the sentence: 'what do you get if you multiply six by nine?' Since six times nine equals 54, not 42, it prompts Arthur to say "I always thought something was fundamentally wrong with the universe".
However, it is implied that this may not be the 'true' Question. The second book also reveals that a group of humanoid aliens called the Golgafrinchans once crash-landed on Earth, 8 million years after its creation. This event may have disturbed Earth's remaining 2 million year of computing, thus producing an incorrect Question.
Interestingly, the first book starts with the following epigraph: 'There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory, which states that this has already happened.' These theories are addressed in the third book, Life, the Universe and Everything, where Arthur and Ford learn of a man called Prak, a witness in a trial who accidentally received a large dose of truth serum, and was then instructed to tell the Truth, the Whole Truth and nothing but the Truth. This caused him to reveal everything, including many secrets of the Universe that people were unprepared to hear. To prevent him from causing insanity in others, he was sealed inside the court room. Hoping that this man may know the Ultimate Question, Arthur and Ford visit his location. Prak confirms that '42' is The Answer, but also asserts that by principle, it is impossible to understand both The Question and The Answer within the same Universe: this would immediately cause the Universe to be replaced by something even more bizarre, and this may even have happened once already.
Apparenty, much of Prak's initial revelations about the Universe involved Arthur Dent, but he has since forgotten their exact contents. The only thing he remembers is the location of God's Last Message to his Creation. Arthur travels there in the fourth book (So Long, and Thanks for all the Fish); the message reads "We apologise for the inconvenience".
Much of this exposition suggests that there is a higher, yet fallible power involved in the creation and control of the Universe. 'God' is occasionally mentioned in the books, as a flawed and somewhat incompetent Creator; at one point, He had a dialogue with man, in which He himself refused to prove his own existence, "for proof denies faith, and without faith, I am nothing." Several other divine-like entities are mentioned in the books as well, usually as obvious parodies of mass religions. So the idea of The Ultimate Question and its Answer may parody the concept of 'God works in mysterious ways' in Christian religion, the notion that all things, for better or worse, happen according to a divine plan that man simply cannot comprehend.
The Answer pops up on several more occasions: in the second book, Arthur finds himself sitting on table 42 in the Restaurant at the End of the Universe, while in the final book, Mostly Harmless, Arthur meets his demise in a club with house number 42, due to the simultaneous destruction of planet Earth in all dimensions. The fact that Arthur Dent seemed to play a part in many of the Universe's secrets is probably relevant to The Question and The Answer, yet in what way remains undisclosed.
Writer Douglas Adams was asked several times about the significance of the number 42, and he gave varying responses, such as that he had to come up with an answer that made no sense at all, and decided it should be a random number; at other instances, he claimed the number had to be completely ordinary and smallish; 42 came to mind, as it was not only divisible by two, but also by six and seven, and "it was the sort of number that you could without any fear introduce to your parents". Later, Adams claimed he got the number from the punchline of a John Cleese sketch, and decided to use it, as 42 was "the funniest of the two-digit numbers". Adams has always rejected all religious or scientific explanations, such as that 42 has significance when written as binary code or in different positional numeral systems. Other theories include that 42 is the average amount of lines per page in paperback books; 42 is the number of laws in cricket, which features often in the books; or that Adams recycled the number from an earlier sketch he wrote, or was influenced by Lewis Carroll, who used the number frequently in his works. But there could well be a simple answer assuming 42 is a code for something: taking A=1 and Z=26, FISH=42. Which could explain the Bable fish, the dolphins, So Long, and Thanks for All the Fish etc.
By contrast, actor and comedian Stephen Fry, who was good friends with Adams, claims that Adams once told him in strictest confidence the exact reason for choosing the number 42: "The answer is fascinating, extraordinary and, when you think hard about it, completely obvious. Nonetheless amazing for that. Remarkable really. But sadly I cannot share it with anyone and the secret must go with me to the grave. Pity, because it explains so much beyond the books. It really does explain the Secret of Life, the Universe, and Everything." It will probably never be clear if Fry is serious, or that he is simply continuing Adams' joke.