switch function in C always showing default case

Question

I'm having trouble with a switch/case statement I created to work as a character check on an input.

My understanding of the switch/case statement is the following

switch (variable) 
{
    case value0:
      Code to execute if variable==value0
      break;
    case value1:
      Code to execute if variable==value1
      break;
    ...
    default:
      Code to execute if variable does not equal the value following any of the cases
      break; // not necessarily needed but good practice
}

So what I came up with for character validation was the following: (sorry the formatting isn't great! validInput is set to 0 before this, I cut some code out to help highlight my issue)

while (validInput==0)           // creates loop awaiting valid input 
{
    scanf("%c",&replay);
    switch (replay)  // BUG TO FIX, displays default text before character entry
    {
    case 'Y':
    case 'y':
    //printf("Yes key pressed\n");  //debug
    turns = lives;
    validInput = 1;
        break;
    case 'N':
    case 'n':
        //printf("No key pressed\n");  //debug
        end = yes;
        validInput = 1;
        break;
    default:
        printf("Invalid input, please try again\n");
        break;
    }
}

What I get each time is

Invalid input, please try again

*Then HERE I can enter the character and the behaviour is as expected. Y (or y) restarts like I want it to and N (or n) exits the game. All other keys show the text twice, I have shown my understanding of the order below:

Invalid input, please try again
l
Invalid input, please try again
Invalid input, please try again
R
Invalid input, please try again <- this is the error message I want to display
Invalid input, please try again <- this is the 'awaiting input' stage...

etc.

Why is it showing the default case at the beginning when it's awaiting the input? I really don't fancy writing an extra long case for all other keys!

EDIT I thought this was a switch/case issue but turns out it was a scanf issue. I didn't know that but thank you to those who helped fix it. My question was given -1 because of this :(