Walter Crabtree was dancer Lola Montez' pianist.
Lola and Walter came to Ajax Camp in California to perform. The miners of the town were possessed by an alien after drinking watered-down whiskey, but Walter was unaffected because he didn't drink whiskey. When Lola wanted to confront the possessed men, Walter stopped her.
When the Sixth Doctor tried to communicate with the alien, Walter and Lola distracted the possessed men by performing for them. (PROSE: Whiskey and Water)