Cruel, as you can see from reading the various posts on this thread, everyone uses a slightly different technique and some use no technique, and merely learn their lines and hit their marks and let the character evolve.
Rather than looking for and using a single technique, if you are a budding actor, I would suggest looking at a lot of methods and trying different things. Then settle on what works for you. I have worked with actors who are in character all the time, from the moment they arrive at the theater until hours after the curtain and some whose families cower in fear of them being cast in a role that will force them to live with a serial killer every day until the show closes.
That technique has never worked for me. But I am obsessive about preparation. I want to know the words very early on and most of the time I know everyone else's dialogue and their blocking. I find that this frees me to explore the character and to try different things and figure out what I want to do long before the director has finished with the other cast members and is ready to freeze the show. So, for me, it is the freedom to explore the character that helps. I also find that mannerisms and even costuming and hair cuts can change my take on things and I look for the right way to use these things to express the character. After all, doesn't the way you dress and groom yourself tell people a bit about you? You don't always have control over these things but you can use what they give you to help create the overall externals. The internals are up to you.
If you are a fan of the 'back story', you can try to create this and your director may want you to do so. I'm not big on the motivation of each line kind of directing but I've had to work with directors who believe in this and it can be rather tortuous to come up with the driving force behind every word you say. Understanding what makes the character tick as a person is a big help in those cases.
Some of the best actors out there will tell you that they learn their lines, hit their mark and let the muse come out.
But, as a very wise, and more experienced actor than I, has often said, you look for what the character 'wants' out of life, out of the situation, in a particular circumstance.
The long and the short of it is that, no matter what method you use to get to your character, you should at least give some thought to the person you are portraying on stage. Don't get too hung up on the words or the emotion you are supposed to be feeling. Understand who you are on the stage and what you want ad your portrayal will be more honest.
And please: try not to anticipate what is about to happen on the stage. Stay in the moment and use the dialogue and the other actors on the stage as it happens.
Understand that if you are a novice, you will be expected to folow the director more slavishly and will probably not get a chance to do much experimentation. That will come with credibility and experience. Be patient and learn from everyone around you. Everyone you work with has something to teach.
"Always smile at your enemies. It will keep them on their toes"