A good fleece sits close through the shoulder and drapes rather than standing away from the body, so when you lift your arm the fabric moves with it. A poor one holds its own shape, bells out at the hem like a tent, and pills into little grey beads within a fortnight of real wear, and I can spot the difference from across a room, long before anyone gets close enough to touch the sleeve.
Fleece doesn't sit well against anything with real structure. Put it under a tailored blazer and the shoulders lose their line completely, the soft bulk pushing through wherever the jacket is trying to hold a shape. It clashes with slippery linings too, the kind you get in a smart mac, because the pile grips and drags rather than letting you slide the coat on in one go. And be careful doubling it up with another fleece, or a boucle knit, in the same outfit, it reads as loungewear fast, comfortable for the sofa, less convincing anywhere you actually leave the house.
It has real limits, and it is worth being honest about them. A fleece is not waterproof, however thick, I found that out properly wet on what should have been a short walk, and a sudden downpour will soak straight through within minutes rather than just damping the surface. It is not much of a wind barrier either, the pile lets air through, which is why it works best as a layer rather than the outer defence on a properly cold day. And it has no place anywhere smart. Even the neatest navy half zip looks like it wandered in from a dog walk if the rest of the room is in coats and proper knitwear.
If you own none, start with a mid weight half zip in a fitted shape rather than a boxy one, something with enough stretch in the fabric that it moves freely when you reach up for a top shelf. Skip the bobbled, high pile versions to begin with, they look cosy on the hanger but shed fluff onto everything else in the wash for the first six months. A simple funnel neck or a plain collar gives you more to work with than anything with a big logo across the chest, which only ever looks right once and then dates the whole thing. For colour, go for the ones that behave like a neutral rather than a statement, a charcoal, a deep bottle green, a soft oatmeal, all of which sit under almost anything without an argument and hide the odd snag or watermark far better than black does in daylight. Over a plain white shirt and the jeans you already wear every week, the right fleece just makes the whole thing look like you planned it.