I’m not going to get into the debate about the rights and wrongs of fat-suits except to agree that many times a fat-suit is used as a cheap sight-gag or for offensive stereotyping.
Sometimes an actress agrees to gain weight for a role, like Renee Zellweger in Bridget Jones’s Diary or Toni Collette in Muriel’s Wedding. But these are the exception – though it does happen more often than one might suppose. But there are many good reasons why an actress or a film-maker might choose the fat-suit and prosthetic makeup route – especially where the fat character is only seen briefly in flashback or where the character changes weight during a movie – if: the schedule doesn’t allow it (actress or filming); there are health concerns relating to a drastic weight change; the character has to be seen at different sizes; the actress is (understandably) unwilling to gain weight she’s probably struggled to lose to get other parts.
A remarkable exception was the Spanish movie Gordos set around a therapy group for people with weight issues: the movie had to be filmed in five distinct stages over ten months as the actors and actresses gained or lost weight to portray their characters.
Here you’ll see some of the effects that were achieved with prosthetic makeup as well as fat-suits. For the fat scenes characters often wear clothes with long sleeves and higher necklines to minimise the prosthetics: but why, oh why, is it that so often the characters have to lose all dress sense and the ability to use makeup when they put on weight?
Of course, some of the characters wearing fat-suits and/or prosthetics ‘overweight’ might well be considered to normal and healthy anywhere other than La La Land.