Hell Bound - S5-E4
Corrected entry: When Spike and Angel are talking in Angel's apartment. Spike sits down on Angel's couch. At that time he hadn't realized that he could "will" himself to touch and affect objects around him. Therefore he would have fallen through the couch, much less make the cushions move when he sat.
Correction: Later on in this episode the "Reaper" explains that he has complete control over how Spike interacts with the living world. This explains how he was talking to Fred earlier in the episode and then suddenly fell through the floor. Or when the Reaper made Spike invisible to everyone around him. The Reaper wanted to mess with his mind by constantly changing the rules like this.