Sometimes sketching is the fastest way to start visualizing layout options for myself or with other people. 

After agreeing on a high-level approach and solidifying feature requirements, I begin producing rough wireframes/low-fidelity mockups to work through step-by-step scenarios and content organization.

For more complex interactions I'll sometimes write quick throw-away prototypes to get a fuller assessment of the flow with visual cues and/or basic animations. At later stages production-level prototyping can become more appropriate for the purpose of exposing technical challenges sooner.