Singers and collaborative pianists: a love and hate relation
Do: Know the printed introduction. Figure out how many measures it is, what it sounds like, how it helps you "hear' your starting note, and what beat in the measure you begin singing on. The better you know the introduction, the more confident you'll be at your first vocal entrance.
Don't: Begin your big audition by walking straight over to the piano and asking the pianist to quietly play the introduction so you can "hear how it goes". The better you know your intro, the better you'll sing the whole song.
Do: Learn how to indicate the tempo if necessary. The pianist can't always read your mind, so you'll make everything a lot easier for both of you if you can give your desired tempo. Either do this by counting 1-2-3-4 in the correct tempo or by singing a few notes (if the melody in fact communicates the tempo in a clear way).
Don't: Answer the pianist's question "How fast does this piece go?" by looking at the sheet music and saying "Quarter note equals 80".
Do: If the pianist starts the song at the wrong tempo, gently but confidently lead them by singing a little faster or slower until it's right. A sensitive accompanist will listen especially closely to you at your first vocal entrance, to make sure that you're both "together." They'll gladly follow you where you want to lead them, once they realise you know what you're doing.
Don't: Start snapping your fingers in a vain effort to correct the tempo. Its' very difficult to tell if a singer wants to speed up or slow down from this anyway, and you'll probably just make a quick enemy of the very person who can help you the most at that moment. Be nice and just sing a little faster or slower. If you're in a practice environment, you can probably get away with stopping the music, smiling and nicely telling the pianist, "I'm sorry, we are performing it at the wrong tempo. Can we take it a little faster (or slower)?". This way you don't embarrass anyone and the pianist will thank you by playing even better than before!
Do: Practice the piece so well that you'll always enter in the correct key.
Don't: Come in a 2nd, 4th, 5th or other interval away from the correct pitch. Learn your key, and pianists everywhere will respect you and want to be your friend.
Do: Make sure your sheet music is clean, easy to read, and in the correct order.
By now I'm sure you're starting to get the idea. Yes, be your wonderful, musical, expressive self. Sing your heart out. Act the role. Show off your high notes. Whatever it is you do, do it well. But at the same time, learn to think like a pianist. Become the conductor too, through your voice and your common sense. The pianist is the collaborator and the person who will help you perform the way you want to perform. The more they enjoy making music with you, the more you'll enjoy making music with them.
Good luck with all your musical endeavours!