How to get into character quickly
You have an audition, or a casting, and you've been waiting in a non-descript room to be seen. There are other actors around - some of them are to be seen for the same role as you. You already know that, so there is no value in analysing how they look/what they're wearing compared to you. You have no control over such things. What you do have control over is your professional approach to getting into character for your audition.
Use this waiting time in a focused way. You know the character; you've done your research. Now is the last moment to immerse yourself into that role. Close your eyes if it helps. Think about the mood of the piece you are going to read for/have prepared. Think about the feeling of the character who is saying those words. Why did he/she choose those words? You may well have learnt them or be familiar with them but what, exactly, is going on behind the words?
Stanislavsky's WHO, WHAT, WHY, WHERE, WHEN questions will help you enormously at this moment. Go over them in your head and provide the answers to yourself all over again, even if you know them already. There is a big story going on around, within and throughout the words of monologue/dialogue. Consider them all. What is your character feeling, during this piece? If you have identified properly with the character, then you can use the words to start believing in your role. You are being auditioned either because you are potentially right for this job or you have chosen this particular audition piece (generally) because it suits you well, as a type and as a professional actor.
Waste no time. Keep your eyes gently closed and be aware of your breathing. Go over and over the words in your head, or snippets of the monologue you have prepared.
WHO is your character? (That doesn't just mean a simple name: it means the background; the thoughts; the circumstances he/she is in.) WHO are you speaking to...?
WHY is your character saying those words?
WHAT is the situation occurring around your character? WHAT are you feeling?
WHERE is the scene situated? Literally, where?
WHEN does this scene occur, in your character's story?
Thsse questions form the fundamentally most crucial aspect of swift preparation. If you identify with the reasons for the words coming from your character's mouth, you will start to immerse yourself into that person's psyche, just by going over the words in your head, in the waiting room. This need take less than 3, 4 or 5 minutes. Everybody's different. But this time is yours to begin inhabiting your role so that, when you walk into that soulless audition room, you are carrying that character within you and nothing else matters.
IN THE DRESSING ROOM / IN THE WINGS
You have had a day that has been very different to your character's. You've had to deal with the children who are playing up / lost the dog / had an upsetting time / been working in a ghastly day job / had a bitter row with your partner / basically, been living your real life. You need to become your character very swiftly.
Take all the steps outlined above but use the luxury of private time to aid you. Close your eyes and breathe easily and normally, but taking notice of your breathing motion for several breaths in and out until you feel calmer. Ask yourself the five W questions and answer each of them, in your head. Leave your real day behind - it doesn't matter any more. It doesn't exist at this moment. You are morphing into your character who has a very different life. WHO..? WHY..? WHAT..? WHERE..? WHEN..? (Personally, the WHY question is the avenue in for me.)
As you answer these fundamental questions in your head, your body will begin to take on the physicality of the character if you are believing in your role. You must identify with your character's situation in every (literal) sense. This is your time. Use snippets of the words that you have been given to provide the situation and feelings that are being embraced and experienced by your character. Keep breathing gently and steadily. The last thing you can add is to say, quietly to yourself, 'My name is .... ' using the name of your character, in his/her voice, before you leave your dressing room or make your entrance.