One of the most difficult things that any actor faces is starting a scene or monologue. Part of the reason that it's difficult is that it comes out of now. Suddenly the lights come up or the film starts rolling and you are expected to be in the moment feeling every emotion to its fullest so that you can showcase that for your audience. However, unless you have a solid grip on the moment before your scene starts your chances of being convincing in the spotlight fall from slim to none.
Expert actors have learned to immerse themselves into the moment before their scene so that you are immediately transported into their frame of mind. The reality is that most scenes start in the middle of a situation that has had a lot of backstory inserted in it. There are relationships that are in place, events that have happened, and feelings that have already been formed. It is understandable that a new actor may find themselves struggling with this, but there are some things that you can do to make it easier for yourself.
In order to truly understand the moment before your scene you need to really explore your characters back story. You need to take the details that you are provided by the script and use those as a springboard to create your characters history. Where did you come from, and how did you get here? Allow your creativity to flow during this process, and focus on the details. You want to be clear on every feeling good or bad that your character has experienced before the lights came up so that you can embody them as you begin your performance.
Additionally, you may want to rely on some techniques to get yourself into the moment of your scene. This can be especially important if you need to dive right into an emotionally charged scene. One thing that may be useful is to use that back story that you had written as something to focus on prior to acting. Perhaps you simply need to re-read it to get yourself in the right place. Or maybe you could do an alternative exercise like writing a letter to the other character that you are interacting with. Doing something like this will help you to get into your characters necessary emotional state.
Another thing that some actors use to get themselves into the moment before is to rely on sense memories. A sense memory is a very vivid memory that you have from your own life where your emotions were very strong. It could be your happiest moment or your saddest. The emotion is not as important as the intensity of that emotion. What you want to do is connect that memory's emotion with the emotion that your character needs to be feeling. Then you try to take yourself back to that moment in your head until you feel that moment just as vividly as you did when the experience first took place. By doing this you can essentially transport yourself to a place of true feeling so that when you start your scene you are instantly emotionally invested.
By doing this you will be able to create a more convincing and interesting character and scene. If you are able to do that you will not only begin nailing your auditions, but you'll soon be getting rave reviews as well!