"Interests are what Sims talk about. To increase a Sim's interest in a particular topic, try reading a magazine."

I'm confused about how to use these interests in my game. Should I make sims take jobs that engage their interests, or jobs for which they are best qualified, or just the jobs for which they roll a hope?

As an example, I have Marsha Bruenig just graduating from college. She is a knowledge sim with secondary family, and her hobby is Science. Her interests are:
Entertainment 8
Paranormal 8
Food 9
Sports 10
School 3
At the moment her LTW is Education, though she has so little interest in School. She did spend some time in the college Art and Craft Department, where she listened to Dora Ottomas standing behind the lectern, and her interest in School rose to 4.

A further example is provided by her bosom chum, Annie. She is a Family sim with secondary Knowledge, and her hobby is Film and Literature. Her interests are:
Politics 7
Food 7
Weather 8
Culture 10
School 4.
She also wants to be a teacher, at the moment. After I recorded those interests, Annie sat down to study Cooking, and these messages popped up:
"Annie has gained enthusiasm in Cuisine! She can now serve cheese platters."
"Bon Appetit, Annie! Your interest in Cuisine is admirable and I would like to heartily extend an invitation to Sue's (Secret) kitchen, a place sure to whet your appetite."
After those messages, Annie's Food interest score was still 7.

Does 'enthusiasm' mean 'interest', or 'skill'? I wonder whether 'Culture' includes music and painting and writing? Perhaps Annie should stay home and write novels? I am looking to enrich my game experience, and to do my best for my sims. Any ideas on how to use interests?

Enthusiasm is to do with hobbies. You can find it under the hobby panel. It's not related to interest, although a sim's one true hobby is determined by their interests.

I sometimes use interests, but in later EPs there are so many other things to pay attention to that I often forget about them. Plus, one of the EPs (freetime, I think?) changes their interests at certain ages to the point that (IMO) they become a bit useless because you can't rely on any consistency over their lifetime. (If anyone knows a mod for this, I would be most grateful - I never found one before.)

I often use them to choose between two jobs which are available to a sim, or to choose which major they might choose.

In theory but I often forget to check - I use the interest in Paranormal to judge how open they are to supernatural sims (a low interest means they are scared of them, a high interest means they are fascinated by them), I use the interest in Animals to see whether they are likely to want to get a pet, or agree to other family members getting a pet. A sim with a high interest in Fashion is likely to be well-dressed, whereas one with a lower interest might not care about their clothes. Sims who like the environment are more likely to be environmentally conscious, Etc etc. I do find when you play to these, your sims seem to gain more personality because their conversations match who they really are rather than just being random. But as said - I often forget to look at them these days because there is too much else to think about.

Enthusiasm is how many points a sim has in a hobby. Enough points and they get a hobby lot card, full points they get the plaque and along the way other things such as blogging etc.
Interests are as they say what a sim talks about but also depending on their personality what they will argue about. A grumpy sim with zero interest in money will have an argument when another sim tries to talk about it. Ending with finders in the ears or stomping off.

With jobs I have two ways to go about things. Either I have a job that my town needs filling and I change a sim to fit the job or I look at the sim and decide their job based on interests, enthusiasm/hobby and sometimes LTW. I do the second once my hood is larger and its not so important to have a teacher, farmer etc. .
For the first sim you could do something with the Paranormal and science side of things such as running a science centre/planetarium or perhaps she might like to be a science teacher. Or you could take the food and sport and say she is into health and open a gym selling salads and healthy foods.

The second sim I would probably go with the culture, film and literature. Could make a good novelist (although maybe not for your game the way it is) A bookshop cafe with a piano player. I had a cafe in game like that once. I use sim wardrobe buffet, sold coffee and the owner played piano.

I don't often give my sims rabbit hole jobs.

Interests are icons what you see in speech bubbles. Sims with a same interests are more likely getting along with talking. If someone who doesn't have interest in theme another sim is talking about, they will get negative outcome. They are very important to build social points and keep two sims talking.

The magazine mentioned here are the base game ones, that you buy from a stand, not the hobby magazines you sign up for over the phone. The only real effect interests have on your Sims is, as mentioned previously, to determine what their one true hobby is, and how they react in conversations with other Sims.

Like simsfreq I use interests to determine things about their personality. High interest in animals I interpret as wanting a pet. A Sim interested in fashion is more likely to buy many clothing options and change often (I don't necessarily think fashion means well-dressed, just an interest in clothes, but perhaps they are interested in sweatpants). Paranormal and sci-fi I interpret as interested in aliens, so they may have alien-posters and a telescope. Health I interpret as either caring to exercise, or wanting to work in health care, depending on other things. Work and money affects how career-focused I interpret the Sim to be. High interest in travel I interpret as a Sim wanting to travel a lot, so they'd be more likely to make use of BV and go on trips. Sports is pretty straight-forward, if they have high interest in Sports they watch the sports channel on TV, might use the FT-sports fields, or throw sports parties. Politics I interpret as being interested in society and social issues, so it might affect what job I get them. School I might use to decide how much a kid/teen studies. Weather, environment, crime, toys, culture and entertainment I have less defined interpretations of. I use them in combination with others to see what kind of pass-time they enjoy, and sometimes what career they use. And I use the collected impression of all interests combined to see what kind of interior decorating that Sim would like.

Wait whut? Not me.

Though interests AFAICS are mainly useful in how they influence sims' relationships with each other - the more they have in common, the easier it is for them to build a relationship because it affects the success of their conversations. I do adjust interests to what I see as appropriate for a sim's personality (not the other way round).

My bad, I meant simsfreq. Not sure how I read that post as being by you, but apparently I did

Yes I have seen this to and most of the time for me if my sims have different interests no mater what I do I will never get them to 3 bolts for each.

Although the wiki for hobbies says that predestined hobby is determined by both personality and interests, in my experience it's almost entirely on personality. I've had lots of cuisine sims who start out with no interest in food and nature sims without interest in the environment. What I have noticed is that sims quickly gain interest in the things related to the hobbies they engage in, especially in an unmodded game.

I started using a mod (tuanisafish's Enthusiasm Overhaul) that affects how interest is gained from hobby activities as I was tired of all my toddlers getting high music and dance enthusiasm and the corresponding interest increase in entertainment and culture just because they knew a nursery rhyme or played with a xylophone. I also roll a ten-sided die to determine a new predestined hobby and use batbox to reset the one assigned to each toddler because I don't want every sim that has a similar personality to also have the same hobby.

I use the interests to narrow the choice of aspiration and secondary aspiration after a sim ages to teen (the sim's interests will change during age up, and I pause the game between that point and the dialog to select aspiration so that I can read their new interest panel). Sims with a high interest in paranormal or school I'm more likely to include knowledge for consideration, while a sim without any real interest in entertainment won't become a pleasure sim.

I also look at interests when choosing what sort of business a sim might start or what community lots they prefer to visit. Mostly though, the interests only really affect how successful "talk" interactions and conversation over meals are as sims with similar interests have better conversations. In the past, when I really want two sims to become friends I tended to avoid "talk" unless I knew they had similar interests. I'm currently using "talk" more as not everyone in real life has only pleasant conversations with everyone, so interests are having more impact on my sim's friendships.

If a sim rolls a LTW that I just don't like, I refer to the interests to see what other LTW (usually career-based) might fit the sim. Other than that, I don't pay much attention to interests. With NL's attraction system and without Freetime, they just don't seem to have much relevance to my game.

in base game, sim interests seem to relate mostly to Chat and Argue.

I use interests to edit hobbies that dont make sense with the sim, and when they are going to get a job. An example would be a sim wanting to be a slacker (usually those romance sims...) but has a high interest in Work, Money and Health and zero on entertaintment or fashion. No, Javier, you're going to be a doctor. My neighborhood needs one and you're the only one with interest in health.(seriously... its just in my games or its coded that way?)
This way you dont end up with all your sims in the same career. Tho I take in consideration their personalities too.
For me: work and money = probably its going to commit to their job and possible promotions, only work = doesnt mind serious jobs, or jobs with long schedules, money= wants to make easy money, or to get a pay thats relatively good to the small amount of hours at work. I would add more but im on my phone >:[
Btw why am i talking about jobs when the topic was about hobbies??

I wonder why I got so many dislikes when nobody else did? Did I give some incorrect info?

Neither me nor my game is influenced by their interests/hobbies much. They all tend to end up having cooking as a hobby and talking about food a lot Which means they dine out a lot and from all the dancing they seem to develop a keen interest in music and dance and then all of them want to buy pianos. They also develop the nature one due to going to fish so that they can learn to make more dishes. And good food and dancing make for great parties
It has absolutely no influence on my choice for their careers either. They will go into the careers my hood needs, or they will go into the military. Or both.

I'm not familiar with the "interests change at a certain point with a certain EP" concept. Perhaps others also weren't, and hit disagree instead of asking or otherwise commenting. (The comment from stitching inclines me to believe that "age up" is perhaps the point to which you were referring).

The only use I find from interests is for social interactions. They can be interpreted any way you want. If they don't seem to be gaining friendship points, the menu has an option to change the subject until you find one that the other sim acts positive to it. If their interests are more difficult to raise points between them, have them interact with objects, such as chess, darts or other group types. Other options show up after awhile for talk about interests or maybe it's share interests. Once their friendship goes to a certain point, it's much easier to gain 3 bolts. I'm unsure if interests count under bolts, only know about aspiration, personality, likes & dislikes.