I did this for a number of years when I was working for some production base and they asked me if I was interested in making some extra cash.. so for about 5 years I worked as one during my days off and weekends.. I have to say I loved it and met some interesting people, from very experienced film extras, crews and a couple of celebs including Ethan Hawke, Robert Downey Jr, Hugh Laurie and Jeremy Irons.. and the money was a bonus.. it can be boring for some.. having to stand in the cold, repeat scenes, wear some dodgy clothes and some extras think they are stars.. but it takes allsorts and it’s definately an experience… and something everyone could do if they really wanted to give it a go.. just be wary of companies offering you work and asking for huge fees to join their casting company but most will charge you admin fees but the ones I used took it out of your first job.