Sometimes saying "yes" is most important as an actor. To explain, this is not about saying "yes" to things that cross over your values, or disturb your integrity, but rather it is an expression of openness.
Saying "yes" is a way to help you move past your fears and make you expand your craft. Think about when you first thought of wanting to be an actor. That took a lot of courage! How many people were behind your choice? In that moment you didn't care who was behind you or not, you just thought of what brings you joy. You chose it whether or not people believed in that choice. You stood up and said "yes" anyway. You said "yes" to your possibilities.
Be open as an artist to things that don't feel safe. The only thing it can do is grow. If in your acting class your teacher wants you to work on a part and your first feeling is fear, just breathe and do the scene anyway. The fear might be that you are being asked to deal with subject matter that might be too close to home. Or you might feel that it is one that you feel you don't "own". Say "yes!". No one ever grew as an actor by staying within their comfort zone. Stretch your ability – especially in a class setting. Who cares if you fall? That is a part of allowing your "yes" and growing
Are you are an actor scared to do improv? Say "yes". In the Improv world, you are only allowed to say "yes". Saying "no" when doing improv stops the scene. Don't stop your very own scene within your career. When you are open to "yes," great work can happen.