First thing's first
One of the best things to do is calculate how long until your production
1) Know how long the Production/Play is for:
EXAMPLE: I did a play for 2 months and I came off script three weeks before the production, however, I made sure I looked at my script at least 2 to 3 times a week to understand my character.
It's always wise to know how long, so you know how much time you have to adapt and learn your lines of your character
2) Read through the whole Script/Play:
No matter how many scenes your character is in, you must read through the whole script. reason being is because you will understand the whole story, and also this will help you understand what relationships you have with the other characters.
You will also improve your character's qualities and purpose in your own scenes.
EXAMPLE: I did a production which was classical theatre and this was very hard, It was very important I read through the whole play to understand the entire story, and because the classical theatre is poetic, I had to analyse and understand what I am also saying when I talk as my character.
3) Understand what you are saying? And why you are saying it
I would say once you have understood the story, you can begin to analyse your own lines. Make notes in your scenes what is happening in the scene and why are you saying what you are saying.
This enables you to feel more confident in what your character is saying, and also identifying in what your character is feeling in your scenes.
4) Knowing your queues:
Once you have understood the whole story you can then learn specifically all the queues before you say your line each time through every scene you're in. this is important because if one performer forgets their line which is your queue you can learn to adjust and improvise your line so the play can run smoothly, also you will be gaining teamwork skills.
So this can be three weeks in of learning your play now you can begin to remember your own queues so you can memorise your queue.
5) Keep on reading, Reflect:
Some scenes may seem more clear to you once a month has passed, so any scenes where you want to be sure, you can always go back and read, read and reflect each time, sometimes the way you say it might change, sometimes you may rephrase what you are saying because you're more in tune with the character you are playing.
This helps with your confidence.
6) Don't be afraid to ask for help:
Anything that you feel will help you on your own, whether that is character profile, getting a friend or another member to read through lines with you, asking your Director for help, going through your lines in the mirror. A warm up.
All these things are valuable through your character development and learning your lines. Keep confidence and keep time schedule.