Some years ago I trained as an RAF selection officer, perhaps the best course I have ever done. The interviewers are trained to grade students, not select them. The interviews are conducted by two interviewers who take it in turns to run the interview, whilst the other takes notes. The first will ask you to describe your life to the present day in chronological order and in some detail, they will want to know what you have done, how well you have done it, and in about 20 minutes will know virtualy everything about you. The second interviewer will probably appear less friendly, he will ask you why you want to join the RAF, and ask you questions that will question your motivation and sincerity. Overall, they are trying to find out what makes you tick and whether or not they would wish to share an aeroplane with you, or live in close proximity with you for long periods of time. You must be honnest or it will soon show, If you don't know things, say so, don't bull****. By all means gain some knowledge of the RAF, where they are based, what they do etc but any last minute cramming will show. You will be asked if you would join the RAF in any other trade if you were not selected as a pilot; they want to know if you want to join the RAF, to be an officer or just be a pilot.
If you get through the medical, aptitude tests and the initial interview, you go on to do practical exercises and problem solving that shows your ability to reason, to lead others, and most importantly to work with and under others.
At the end of the selection procedure you are graded. Whether you are selected or not then depends upon how many people they need and how many they have in the higher grades. You then either get a letter of selection with an offer; a rejection with perhaps the suggestion of coming back for another go, that usually indicates you have made a reasonable grade; or a flat rejection.
The best ploy is to be totally honest and enjoy it, it would be a terrible shame to be rejected for trying to create an impression that they don't want to see. Believe me, they will know more about you than you do by the end of the selection procedure and they know what they are looking for.