If you apply to be a cosigner for someone and never actually sign the lease, you only fill out/sign the application are you still responsible for missed rent or damages?

You signed the application, so in theory, you are agreeing to the terms of the rental agreement. You should be more careful and always read the small print, tanny.

Even if you didn't sign the actual lease agreement, you signed the application so they can really get you on technicalities.
Tick

Often the co/signer does not sign the lease, they merely sign the application, and agree to pay for the lease if the other party does not.

The exact wording of what you signed will make the difference.

But this is what a co signer does, pay the money if the other person does not

If the lease was eventually signed by both the LL and your friend you are now responsible for any unpaid rent or damage. Did you friend sign the lease? Your signature just legally states you will be responsible since he isn't quite there on his own.