How to get the most out of castings
First and foremost, preparation is key. For a musical theatre audition, don't choose a song at the last minute, but spend a period of time looking at your options. Ideally, it should be something that displays a range of emotions, but even songs which may initially suggest a single emotion such as utter despair (e.g. 'On My Own' from 'Les Miserables') can contain pockets of variation. For example, when Eponine is imagining Marius' arms around her, this is a moment of comfort. Similarly, a repeat is never just a repeat; each repetition of a line can be approached from a different perspective as the character thinks through their predicament.
To get the most out of a casting, it is mandatory to know what to expect once you get in the room. Little things can instantly unnerve performers and make them fall to pieces, for example the pianist playing either too fast or too slow. If this happens, just come in at your own tempo and, if the pianist is doing their job probably, they should take note of the speed you are going and play with you. If they don't, do not under any circumstances pull faces or keep staring at the pianist, but keep calm and continue singing and the panel will know that the tempo was an issue. Also concerning the piano, it is invaluable to have a quick word with the pianist before you start singing to give them an idea of how you would like it played and to draw his/her attention to any cuts in the music. These cuts should also be marked clearly on the score.
In order to impress in a casting, it is important to try to hide any nerves you may have. Therefore, I would suggest not taking any props into the audition because they can show if your hands are shaking, particularly if it is a piece of paper. Although the piece you are doing may require props, think very carefully about what you will do with it throughout the whole song,as items can become very awkward if you don't have anything to do with it or a place to put it down.