Why does the same mutation kill one person but not another?

Why does the same mutation kill one person but not another?: The vast majority of genetic disorders (schizophrenia or breast cancer, for example) have different effects in different people. Moreover, an individual carrying certain mutations can develop a disease, whereas another one with the same mutations may not. This holds true even when comparing two identical twins who have identical genomes. But why does the same mutation have different effects in different individuals?