No matter how small your role is you are there for a purpose. You have a role to create and a director to please. You need to really delve and research into your character to create who they are in you. Get an idea of their mannerisms, how they would sit or walk and speak. How do they react to others. Who do they like and who do they dislike. Who do they have feelings for. You need to make this character you are creating believable.
If you aren't' sure what your director wants ask. Tell your director that you are going to try a few things and if they don't like what you are doing to tell you. In this way your director feels like they are helping you create your character and they feel that they still have the control.
When you go on stage its not you up there it is that character that you have created. A good stage actor can take any part no matter how small and make it their own.
Create the character. Is he middle aged then he would be huffing and puffing as he struggled up stairs. What is there about the character that makes them unique?
If you don't have any lines you can convey your thoughts by using your face to show the emotion you feel. If you going to far overboard your director will tell you and bring you back in where they want you.
If somehow by accident something happens on stage which will not take away from the play use it! I was in a play and the wrapping strings on the packages I was delivering got caught on my delivery coat button. When I dropped the packages as I was supposed to do they just hung from my coat. Would you panic? Run out of the scene in terror? NO use this situation to your advantage. Stay in character but use it.
If you are singing in a show practice, practice, practice! Your voice is your instrument and without it you can't perform.
Take care of your voice. Drink lots of echinea & Honey tea before you sing. You can find this in about any health food store. The hot tea lubricates your throat and loosens up your vocal cords keeping you from a scratchy throat. You also will be more easily able to hit those high notes.
NEVER BREAK CHARACTER unless your director says that it is ok. Breaking character is a cardinal sin. You are in the play. That is your real world from the moment the curtain goes up until your final bow. That world on stage is populated with people that you react with and to. The audience is watching what you will do next. They are observers of your world. They want to believe. Never do anything to bring them back to realizing this is a play.
If you have a small part in a play with just a few seconds on stage do you want "pity applause"? That is what you will get if you don't do your job on stage well. You have only a few seconds on stage then make that time memorable. You can tell if you are receiving pity applause because the audience response sounds very weak and hollow and forced. The audience is trying to be polite but they probably don't even remember what you did in the play. What You want to hear is a rounding applause from the heart of the audience.
If you are ever on stage and hear that sound you will never get it out of your head. And you will always strive not to hear it again.
Mistakes happen during live shows. Cast members forget their lines and cues. It's up to you to keep the play going. Know your part and also keep track of what the others actors are doing. If possible know their parts too so you can step in if necessary to keep the play going.
Be ready to feed a line to an actor who has lost their concentration. Things do happen even in the most rehearsed stage production so its how you handle them that counts. Be on your toes!