Use other account's files from my account?

I am something of an administrator on this computer. Therefore I know all of the passwords.

Sometimes it would be useful if I could look at another user's files from my account. BUT I don't want to normally have access to these files -- I only want to have access to them after I have entered the password.

I could easily login to their account, do what I need to do, then log back into my account. It would be easiest if I could just do everything from my single account.

I know how to open a new Finder with root access, but when I try opening the files (e.g. pictures with Preview) it doesn't work because I technically don't have the access privileges. Does anybody know of a way for me to accomplish what I am trying to do?

I have also tried connecting to their account as a server but this isn't convenient either. I just want to enter a password, have access to all of the files FROM MY OWN ACCOUNT, then logout (this includes viewing, opening, etc.) Thanks!