How did you choose your effects?

Buck, when working out your parts for the individual songs, especially in the Agents and Spectres era, how did you determine whether you'd use a particular tone or effect, such as the wah on Tenderloin and Searchin for Celine, and the phase on Nosferatu? Was it experimenting with sounds until you found what fit, or was it more instinctual, like you could hear what it needed beforehand?