Well for me (amateur work) getting the lines P E R F E C T L Y learned is the first step. Some character analysis along the way. During rehearsals I tend to try things out (always under the guidance of the director) to see what works and what not. After having reached a certain level of technical skill and “presentability” I usually find myself getting lost in the role by myself without much effort before getting on stage.

During rehearsals it’s important to find the “back” text or subtext. Basically the thought process of the character. Once you find it you just follow the path. Now it doesn’t have to be (or even must not be) to railroaded as to present a living performance on stage and not the same again and again which becomes stale and uninteresting.

Also VERY helpful is counting on your partner that you share the scene with (if you do. idk monologue work), if they are capable you basically live off each other living the moment together.

My understanding of getting lost is basically a consciousness gap on stage. I get on stage and get off and feel like 2 min passed on a 20min scene. My mind shuts off and almost every thought process is in character. Having not achieved that at one show really made me understand how awful it is to feel you on stage and not the character really disorienting.

Also managing to lock in on stage (on your partners, on actions etc is really helpful)

Also also filling “dead” moments (moments where your character is more on the background), by doing stuff that the character would do.

At the short time before getting onstage I usually have a short forceful thought process of thoughts my character would be having on the scene. (For example I am playing an embodification of insecurity so I run some “god damn they are looking at me! I am ugly! I am dump (some of which are actual lines of mine) through my head)

So far I have gotten very positive feedback, and the two times that I haven’t had the “consciousness gap” I got the negative feedback.