Thecodont, the difference is that interest exp is a cash expense while depreciation is not.
You are correct. Essentially depreciation/amortization you can think of getting the total amount back (added back to arrive at FCFF) and a bonus which is related/equal to its taxable amount. So yes, you get Dep + (Dep x T) essentially.
Interest on the other hand is being added back not becuase it’s a noncash expense but bc we are trying to get to FCFF (total debt and equity), so we are only adding back the piece of int exp that is essentially missing which is Int (1-T). This we subtract out later to arrive at FCFE. Because interest is a cash expense, the only benefit you receive is (Int Exp x T), not this plus all of interest like was the case with depreciation.
That’s about as clear as mud, but hopefully that helps somewhat.