River Phoenix running down the street after he left Rose was the last scene shot and was not intended. It was added last minute when everyone noticed the skyline. River was told to run and didn't know why. He laughed through the whole scene because everyone yelled "Run River Run!"
The first and last scenes (River Phoenix on the bus and then River and Rose reuniting) were both shot the first day of filming. River was upset about this unusual way of movie making because he had to shoot the last scene before making the whole movie.
The character of Rose was originally supposed to be markedly overweight, but Lili Taylor made such an impression on director Nancy Savoca that the diminutive actress eventually won the part. Taylor did, however, gain 12 lbs. for the role and wore padding underneath her clothing.
To prepare to play the Four B's, a close group of marines, River Phoenix (Birdlace), Richard Panebianco (Berzin), Mitchell Whitfield (Benjamin), and Anthony Clark (Oakie) were sent for a week of simulated boot camp, run by retired U.S. Marine Corps Captain Dale Dye.