The scenes with Bruce Lee's spirit were actually filmed with Tai Chung Kim and Kurt McKinney both speaking different languages. Kim's voice was later dubbed into English and, apparently, both actors did not know what they were saying to each other throughout these scenes - they spoke from cues behind the cameras.
The scene that has Kurt McKinney doing two-finger push-ups was actually filmed with wires to hoist McKinney up and down. They used trees in the background to cover up the wire.