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Actors don't read scripts like your average person does. Most people only read scripts with the intent of getting entertained and likely never read it again. An actor might be entertained on the first read but then immediately has to read it over and over. Actors get a sense of the scene from their first reading but subsequent readings are for getting more from it.

Look For Objective

Your character wants something in this scene you're reading. It may not be obvious, but it's there. Read it until an idea comes to you and then read it again with a possible objective in mind. Look for clues not only in what your character does and says but also how the other characters react. Don't stick too close to the exact text of what they are saying and doing but rather think about what they would use those words for. What do they stand to gain from a line? Is there a pattern being set from line to line?

What Is The Action Of Each Line

Each line is not just something they say, it's an attempt to get something with the least effort possible. If you aren't clear on what they are trying to say then you may deliver a line wrong. Look behind the text like you did when looking for an objective in the scene and give it a strong verb like "to embarrass him". If you weren't able to get a good objective at first, breaking down behavior and dialogue into the actions behind them may help.

What Physical Actions To Add

Once you know what your character wants and what each line and specified physical movement means, then you can go about adding more to help your character reach his or her goals. If your character's actions are "to scare" as opposed to "to seduce" you will use different vocal delivery and stage business. When you want to scare you might raise your voice and use strong body language, when seducing you might lower you voice to a whisper and lightly caress the other's arms.

Where Are Transitions, High Points

Transitions are the parts of the scene where your characters feelings drastically change and are often preceded by a high point of emotion. These can help you temper the way you deliver lines up to and after these points. Transitions are a soft version of acts. They break down the scene into parts and give it a dynamic quality. Since your character wants something and either gets it or doesn't, you can see how that would affect the character's emotions and prompt some kind of change.

Progressions, Variety Of Delivery

Characters don't use their best material at first. They are never the maddest or most charming at the beginning of a scene nor do they stay that way for the remainder. People don't expose themselves needlessly and neither will characters. So in your scene keep in mind that there will be a journey of effort and risk. As each line is unable to get them what they want they will need to add effort and risk more. As things progress they will need to try new things each time.

Character Clues

Sometimes who the character is just doesn't come across at first. The character may seem boring or to lack dimension. The script can help you figure out who your character is. Read closely for choices the character makes to help you perform a scene better. For instance the words the character chooses can help you come up with who they are. Articulate lines with perfect grammar may seem to come from a stodgy character, but if you added a stutter then you can open the possibilities. A character that only speaks in short sentences may seem to be stupid but maybe they are doing this to hide their true identity.

The script doesn't completely spell out everything but rather is a map for you to explore. You have a lot of leeway when creating characters and the only thing you need to reconcile with is the script and director. Use the script as a guide, but don't stop there. A dose of imagination could help you bring a decent script to the level of greatness (or at least good).