fill-in /ˈfɪlˌɪn/
  臨時填補空缺的人,臨時填補物(a.)臨時填補性的
  fill-in
       n : someone who takes the place of another (as when things get
           dangerous or difficult); "the star had a stand-in for
           dangerous scenes"; "we need extra employees for summer
           fill-ins" [syn: stand-in, substitute, relief, reliever,
            backup, backup man]