Last year I was invited to play Shostakovitch's second piano concerto. The orchestra was doing a programme of classical music used in the movies, with half of it dedicated to Disney's original Fantasia and the 2000 version. About a month before the concert, the conductor called me up.
Due to that concert being done by a guest conductor, there were going to be some changes to the programme. "Can you play Gershwin's Rhapsody in Blue?" he said. I had never played the Rhapsody before, but back in high school I'd played around with the score a few times. I was pretty sure that I could learn it in five weeks, before the rehearsals with the orchestra started, but I decided it was better to make sure and play through the score once. "I'm not sure," I said, "let me look at the score and I'll get back to you in about an hour."
I sat down and played through it, didn't find anything that was really hard to play and saw it was pretty straightforward to memorize. Most of it is also played solo and very improvisatory, so there was a lot of freedom to move around. Just as I suspected, I could easily play it by then. I called the conductor back in 40 minutes to tell him that, only to have him say: "Oh, I'm sorry, we already got another pianist to play it. We'll do Shostakovitch with you next season."
This is a very competitive field. I was lucky to get a shot at playing it and forgot that there were a dozen other pianists ready to play it in case I wasn't; and that's just locally. Two weeks later I got another invitation to play Rhapsody in Blue with even less time to learn it. I said "Yes. No problem." on the spot. I did what I had to do to get the piece ready and a couple of suitable encores (I prepared Gershwin's Lullaby in Blue and a short piece by Kapustin) and the concert and rehearsals went very well.
If you are one of those pianists that always says "I'm not sure," "Maybe," "Who knows" in answer to the question: "Can you play this?" you need to learn to say: "Yes" or "No" because it is nobody's problem but your own whether you can learn a piece on time or not. I've written about this before on this page: one of the most important things a musician must have is a realistic view of what he can and can't do.
As a student, a lot of the repertoire we play is designed to overcome hurdles, to learn to do things we hadn't done before. Our attitude when faced with a new piece is usually one of "I'm not sure if I'll be able to play this." Because of this, it's easy to fall into the habit of prefacing everything we do with an excuse. Excuses are just a projection of our own insecurity. They can ruin your performance because you end up sounding as if you are apologizing for playing. The "student" attitude also can be terrible for your own self-worth. As we gain maturity and technical mastery, that attitude has to change. When learning a new piece it's not about whether you can or can't play it, it's about when. You have to do what you can to make it good, and that is nobody's problem but your own.
The manager, the concert promoter, the conductor, the old lady from the music society that hired you to play the recital, the people in the audience; they don't care how much you had to practice-- or how little-- or whether its the first time you play it, or if you get nervous in public, or if you don't play the really hard passage in measure 243 perfectly clean. All they want is to hear a piece of music. To be, for a moment, lifted from every day's routine by art, beauty and emotion.
Don't give excuses.
Just say "Yes, absolutely," or don't do it at all.